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THE    CONTEMPORARY  SCIENCE   SERIES. 
Edited  by  HAVELOCK   ELLIS. 


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THE    EVOLUTION    OF    SEX. 


THE.  -.    .(-^^^ 

Evolution  of  Sex)^^^ 


BY 

Professor    PATRICK    GEDDES 

AND 

J.    ARTHUR    THOMSON. 


With  104  Illustrations, 


SCRIBNER     &    WELFORD, 

743    &    745    BROADWAY, 

NEW    YORK. 

1890. 


PREFACE. 


IN  course  of  the  preparation  of  critical  summaries,  such  as  the 
articles  "  Reproduction  "  or  "  Sex,"  contributed  by  one  of 
us  to  the  "  Encyclopaedia  Britannica,"  or  the  account  of  recent 
progress  annually  prepared  for  the  Zoological  Record  by  the 
other,  we  have  not  only  naturally  accumulated  considerable 
material  towards  a  general  theory  of  the  subject,  but  have  come 
to  take  up  an  altered  and  unconventional  view  upon  the  general 
questions  of  biology,  particularly  upon  that  of  the  factors  of 
organic  evolution.  Hence  this  little  book  has  the  difficult  task 
of  inviting  the  criticism  of  the  biological  student,  although 
primarily  addressing  itself  to  the  general  reader  or  beginner. 
The  specialist  therefore  must  not  expect  exhaustiveness,  despite 
a  good  deal  of  small  type  and  bibliography,  over  which  other 
readers  (for  whose  sakes  technicalities  have  also  been  kept 
down  as  much  as  possible)  may  lightly  skim. 

Our  central  thesis  has  been,  in  the  first  place,  to  present  an 
outline  of  the  main  processes  for  the  continuance  of  organic 
life  with  such  unity  as  our  present  knowledge  renders  possible  ; 
and  in  the  second,  to  point  the  way  towards  the  interpretation 
of  these  processes  in  those  ultimate  biological  terms  which 
physiologists  are  already  reaching  as  regards  the  functions  of 
individual  life, — those  of  the  constructive  and  destructive 
changes  (anabolism  and  katabolism)  of  living  matter  or  proto- 
plasm. 


VI  PREFACE. 

But  while  Books  I.  and  II.  are  thus  the  more  important, 
and  such  chapters  as  "  Hermaphroditism,"  "  Parthenogenesis," 
"  Alternation  of  Generations,"  have  only  a  subordinate  and 
comparatively  technical  interest,  it  will  be  seen  that  our  theme 
raises  nearly  all  the  burning  questions  of  biology.     Hence,  for 
instance,  a  running  discussion  and  criticism  of  the  speculative 
views  of  Professor  Weismann,  to  which  their  very  recent  intro- 
duction to  English  readers^  has  awakened  so  wide  an  interest. 
At  once  of  less  technical  difficulty,  and  in  some  respects  even 
wider  issues,  is  the  discussion  of  Mr  Darwin's  theory  of  sexual 
selection,  reopened  by  the  other  leading  contribution  to  the 
year's  biological  literature  which  we  owe  to  Mr  Alfred  Russel 
Wallace.!    Besides  entering  this  controversy  at  the  outset  of  the 
volume,  we  have  in  the  sequel  attempted  to  show  that  the  view 
taken  of  the  processes  concerned  with  the  maintenance  of  the 
species  leads  necessarily  to  a  profound  alteration  of  our  views 
regarding  its  origin,    although   the  vast  problems   thus    raised 
necessarily  remain  open  for  fuller  separate  treatment.    It  is  right, 
however,  to  say  that  the  restatement  of  the  theory  of  organic 
evolution,  for  which  we  here  seek  to  prepare  (that  not  of  indefi- 
nite but  definite  variation,  with  progress  and  survival  essen- 
tially through  the   subordination    of  individual    struggle   and 
development  to  species-maintaining  ends),  leads  us  frankly  to 
face  the  responsibility  of  thus  popularising  a  field  of  natural 
knowledge  from  which  there  are  so  many  superficial  reasons  to 
shrink,    and    which    knowledge  and   ignorance    so   commonly 
conspire  to  veil.     For  if  not  only  the  utmost  degeneracy  be 
manifestly  connected  with  the  continuance  of  organic  species, 
but  also  the  highest  progress  and  blossoming  of  Hfe  in  all  its 
forms,  of  man  or  beast  or  flower,  it  becomes  the  first  practical 


Heredity."      Oxford,  1889.  f "  Darwinism."     Lond.  1889. 


PREFACE.  Vll 

application  of  biological  science  not  only  to  investigate  and 
map  out  these  two  paths  of  organic  progress,  but  to  illuminate 
them.  Hence  we  have  attempted  to  indicate  the  application  of 
the  general  organic  survey,  which  has  been  our  main  theme,  to 
such  questions  as  those  of  human  population  and  progress, 
although  here,  more  even  than  elsewhere,  our  treatment  can  be 
at  best  only  suggestive,  not  exhaustive.  While  limits  of  space 
have  made  it  impossible  to  give  the  botanical  side  of  our  sub- 
ject its  proportionate  share  of  attention,  our  illustrations  of  the 
essential  facts  are  sufficient  to  show  the  parallelism  of  the 
reproductive  processes  throughout  nature. 

It  remains  to  express  our  thanks  to  Professor  F.  Jeffrey 
Bell  for  some  valuable  suggestions  wh^le  the  work  was  passing 
through  the  press  ;  to  Mr  G.  F.  Scott-Elliot  for  assistance  in 
summarising  certain  portions  of  the  literature ;  and  to  our 
engravers,  Messrs  Harry  S.  Percy,  F.  V.  M'Combie,  and  G. 
A.  Morison,  especially  to  the  first-named,  who  has  executed 
the  great  majority  of  our  illustrations  with  much  care  and  skill. 

PATRICK  GEDDES. 

J.  ARTHUR  THOMSON. 


ERRATUM. 

On  page  146,   instead  of  present  title   of  figure,   read  :- 
The  Process  of  Fertilisation. — After  Boveri." 


CONTENTS. 


I'AGE 
3-15 


BOOK  I.— MALE  AND  FEMALE. 


CHAPTER  I. 

The  vSexes  and  Sexual  Selection 

§  I.   Primary'  and  secondary  sexual  characters. 
§  2.   Illustrations  from  Darwin. 
§  3.   Darwin's  explanation  by  sexual  selection. 
§  4.   Criticisms  of  sexual  selection — 

(a.)  Wallace. 

(/>.)  Brooks. 

{c.)  St  George  Mivart. 

{d. )  Others. 


CHAPTER  H. 

The  Sexes,  and  Criticism  of  Sexual  Selection      -  -       16-31 

§  I.  Search  for  a  broader  basis, 

^  2.   Differences  in  general  habit,  &c.     Males  active,  females 

passive. 
§  3.   Differences    in    size.       Males   smaller,    females   larger. 

Pigmies  and  exceptions. 
§  4.   vSecondary   differences    in    colour,     skin,     &c.       Males 

katabolic,  females  anabolic. 
§  5.   Sexual  selection  :  Its  limits  as  an  explanation. 

Postulate  of  extreme   cesthelic    sensi- 
tiveness. 
Darwin  and    Wallace   combined    and 

supplemented. 
Sexual  selection  a  minor    accelerant, 
natural  selection  a  retarding  action, 
on  constitutional  difil'erentiation. 


X  CONTENTS. 

CHAPTER  III. 

I'AGE 

The    Determination    of  Sex    (Hypotheses    and    Obser- 
vations)    .-.-.--      32-40 

§  I.  The    period    at   which    sex   is  settled.     Ploss,    Sutton, 

Laulanie,  &c. 
§  2.  Over  five  hundred  theories  suggested  — 
Theological. 
Metaphysical. 

Statistical  and  hypothetical. 
•    Experimental.     (Chap.  IV.) 
§  3.  Theory  of  male  and  female  ova  requires  analysis. 
§  4.  Theory    of    "  polyspermy,"    or    multiple    fertilisation, 

dismissed. 
§  5.   The    theory    of    age    of    elements    allowed.       Thury, 

Hensen,  &c. 
§  6.  Theory  of  parental  age  of  secondary  moment.     Hofacker 

and  Sadler. 
§  7.  Theories  of  "  comparative  vigour,"  &c.,  require  analysis. 
§  8.  Theory  of  Starkweather, — many  factors  combined  under 

"  superiority." 
§  9.   Darwin's  position. 
§  10.   Diising's  synthetic  treatment,  and    theory  of  self-regu- 
lation of  numbers. 
§  II.  The  sexes  of  twins. 

CHAPTER  IV. 

The  Determination  of  Sex  (Constructive  Treatment)     -      41-54 

§  I.   Nutrition    as  a  factor   determining   sex.        Favourable 
nutrition  tends  to  females. 
(a.)  Yung's  tadpoles. 
(/a)  Cases  of  bees, 
(f.)  Von  vSiebold's  observations. 
{(/.)  Case  of  aphides. 
(e.)   Caterpillars. 
(/. )  Crustaceans. 
{g.)  Mammals. 
{A.)  Human  species. 
{i.)   Plants. 
§  2.  Temperature  as  a  factor.     P'avourable  conditions  tend  to 

females. 
§  3.   Summary  of  factors  : — 

(a.)  Nutrition,  age,  &c. ,  of  parents  affecting — 
(d.)  Condition  of  sex  cells,  followed  by  — 
[c.)   Environment  of  embryo. 
§  4.  General  conclusion  : — Anabolic  conditions  favour    pro- 
duction of  females,  katabolic  conditions  males. 
§  5.   Hence  corroboration   of  conclusion  of  Chap.    II.,    that 
females     were      preponderatingly     anabolic,      males 
katabolic. 
§  6.   Note  on  Weismann's  theory  of  heredity. 


CONTENTS.  XI 


BOOK  II.— ANALYSIS  OF  SEX— ORGANS,  TISSUES, 

CELLS. 

CHAPTER  V. 

PAGE 

Sexual  Organs  anm)  Tissues     -----       57-64 
§  I.   Essential  sexual  organs  of  animals. 
§  2.  Associated  ducts. 
§  3.   Origin  of  yolk-glands,  &c. 
§  4.   Organs  auxiliary  to  impregnation. 
§  5-   Egg-laying  organs. 
§  6.  Brood-pouches. 

CHAPTER  VI. 
Hermaphroditism  -.-..-      65-80 

§  I.   Definition  of  hermaphroditism  ;  its  varied  forms. 
§  2.   Embryonic  hermaphroditism.    Ploss,  Laulanie,  Sutton. 
§  3.   Casual  or  abnormal  hermaphroditism,  from  jelly-fish  to 

mammal. 
§  4.   Partial  hermaphroditism,  from  butterflies  to  birds. 
§  5.   Normal  adult  hermaphroditism,  from  sponges  to  toads. 
§  6.   Degrees  of  normal  hermaphroditism. 
§  7.   Self-fertilisation  and  its  preventives. 
§  8.  Complemental  males — cirripedes  and  jSIyzostomata. 
§  9.  Conditions   of    hermaphroditism ;    its    association    with 

passivity  and  parasitism. 
§  10.   Origin    of  hermaphroditism  ;    the   primitive  condition  ; 

persistence  and  reversion. 

CHAPTER  VII. 

The  Sex-Elements  (General  and  Historical)         -  -      81-96 

§  I.  The  ovum-theory. 

§  2.   The  history  of  embryology,  "evolution"  and  "epigenesis." 
Harvey's    epigenesis    and     prevision    of    ovum- 
theory. 
Malpighi  and  early  observers. 
Preformation   school;  "evolution"  according  to 
Haller,  Bonnet,   and   Buffon  ;    ovists  and  ani- 
malculists. 
Wolffs  demonstration  of  epigenesis. 
Wolffs  successors. 
§  3.   The  cell-theory. 
§  4.  The  protoplasmic  movement. 
§  5.   Protozoa  contrasted  with  Metazoa  ;  the  making   of  the 

"  body." 
§  6.  General  origin  of  the  sex-cells  in  sponges. 

coelenterates. 
other  Metazoa. 
§  7.   Early  separation  of  the  sex-cells  in  a  minority  of  cases. 


Xll  CONTENTS. 

PAGE 

§8.    "Body"  versus  reproductive  cells,  and   the  continuity 
of  the  latter. 

Owen. 

Hoeckel. 

Rauber. 

Brooks. 

Jager. 

Galton. 

Nussbaum. 
§  9.   Weismann's  theory  of  the  continuity  of  the  germ-plasma. 

CHAPTER  VIII. 
The  Egg-Cell  or  Ovum  .....    97-108 

§  I.   Structure  of  ovum — 

Cell-substance  and  protoplasm. 

Nucleus  and  chromatin. 
§2.   Growth  of  ovum  — 

Transition  from  amoeboid  to  encysted  phase. 
§  3.  The  yolk- 
Its  threefold  mode  of  origin. 

Its  diffuse,  polar,  or  central  disposition. 

Resulting  influence  on  segmentation. 
4.   Composite  ova. 
§  5*   Egg-envelopes — 

((/.)  From  ovum  itself. 

{b.)   P'rom  surrounding  cells. 

(f. )    From  special  glands. 
§  6.   Birds'  eggs — 

Concrete  illustration  of  facts  and  problems. 
§  7.   Chemistry  of  the  ovum — 

Its  capital  of  anastates. 
§  8.   Maturation  of  ovum — 

Occurrence,  formation,  history  of  polar  globules  ; 
parthenogenetic  ova. 
§  9.   Theories  of  polar  globules — 

1.  Minot,  Balfour,  Van  Beneden,  &c. 

2.  Biltschli,  Hertwig,  Boveri,  i\:c. 

3.  Weismann. 

CHAPTER  IX. 

The  Male-Cell  OR  Sperm         .....  109-116 

§  I.   General  contrast  between  sperm  and  ovum — 

An  index  to  contrast  between  male  and  female. 
§2.    History  of  discovery — 

(a.)  Hamm  and  Leuwenhoek. 

{/^)  Animalculists. 

(<:. )   Classed  as  Entozoa  or  parasites. 

(^/. )  Kolliker's  demonstration  of  cellular  origin. 


CONTENTS.  Xlll 

§  3.   Structure  of  sperm —  i'age 

_  ''  Head,"  "  tail,"  "middle  portion,"  &c. 
§  4.    Physiology  of  sperm — 

Locomotor  energy  and  persistent  vitality. 
§5.   Origin  of  sperm — 

Theory  of  spermatogenesis. 
§  6.   Further  comparison  of  sperm  and  ovum — 

Processes   comparable   with    formation    of   polar 
globules. 
§  7.   Chemistry  of  the  sperm. 


CHAPTER  X. 

Theory  of  Sex  :  Its  Nature  and  Origin     -  -  -     11 7-134 

§.  I.   Suggested  theories  of  male  and  female — 

Rolph. 

Minot. 

Brooks. 
§  2.   Nature  of  sex — seen  in  Sex-cells. 

The  cell-cycle. 
Protoplasmic  interpretation. 
§  3.   Problem  of  origin  of  sex. 
§  4.   Incipient  sex  among  plants. 
§  5.   Incipient  sex  among  animals. 
§  6.   Corroborative  illustrations. 
§  7.    General  conclusions  from  foregoing  chapters. 


BOOK  III.— PROCESSES  OK  REPRODUCTION. 

CHAPTER  XL 

Sexual  Reproduction  .  .  .  .  -     137-156 

§  I.   Different  modes  of  reproduction. 

§  2.   Facts  involved  in  sexual  reproduction. 

§  3.   Fertilisation  in  plants — 

P'rom  Sprengel  to  Strasburger. 
§  4.   Fertilisation  in  higher  animals — 

From   Martin  Barry  and  Biitschli,  to  Van  Bene- 
den  and  Boveri. 
§  5.  Fertilisation  in  Protozoa. 
§  6.  Origin  of  fertilisation — 

{a.)  Plasmodium. 

(d.)   Multiple  conjugation. 

{c.)   Ordinary  conjugation. 

{d.)  Union  of  incipiently  dimorphic  cells. 

{e.)  Fertilisation  by  differentiated  sex-cells. 
§  7.   Hybridisation  in  animals  and  plants. 


XIV  CONTENTS. 

CHAPTER  XII. 

PAGE 

Theory  of  Fertilisation        .  .  .  .  .     157-168 

§  I.   Old  theories — 

{a)   Ovists,     {/?)    animalculists,    (<:)     the     "  aura 
seDiinalisy 
§  2.   Modern  morphological  theories — 

(a.)  Nuclei     all  -  important.       Hertwig,      Stras- 

burger,  &c. 
{b.)  Cell-substance  also  important.      Nussbaum, 
Boveri,  &c. 
§  3.   Modern  physiological  theories — 

Sachs,  De  Bary,  Marshall  Ward,  (S:c. 
Cienkowski  and  Rolph, 
Weismann's  view. 

Critique  and  statement  of  present  theory. 
§  4.   Use  of  fertilisation  to  the  species — 
(a.)  Rejuvenescence — 

Van  Beneden  and  Blltschli. 
Galton  and  Hensen. 
Weismann's  critique. 
'{b.)  The  observations  of  Maupas. 
{c.)  A  source  of  variation.   Brooks  and  Weismann. 

CHAPTER  XIII. 

Degenerate  Sexual  Reproduction  or  Parthenogenesis      169-187 
§1.   History  of  discovery. 
§  2.   Degrees  of  parthenogenesis — 

Artificial,   pathological,    occasional,    partial,    sea- 
sonal, total. 
§  3.   Occurrence  in  animals — 

Rotifers,  crustaceans,  insects. 
§  4.   Occurrence  in  plants — 

Phanerogams  and  fungi. 
§  5.  The  offspring  of  parthenogenesis. 
§  6.   Effects  on  the  species. 
§  7.   Peculiarities  of  parthenogenetic  ova- — 

Weismann's  discovery. 
§  8.   Theory  of  parthenogenesis — 

Minot  and  Balfour. 

Rol]5h  and  Strasburgcr. 

Weismann. 

The  present. 
§  9.   Origin  of  parthenogenesis. 
I  o.  Case  of  bees. 

CHAPTER  XIV. 
Asexual  Reproduction  .  .  .  .  .     188-199 

§  I.   Artificial  division. 
§  2.   Regeneration. 

§  3.   Degrees  of  asexual  reproduction. 
§  4.  Asexual  reproduction  in  plants  and  animals. 


CONTENTS.  XV 

CHAPTER  XV. 

PAGE 

Alternation  of  Generations  ....    200-215 

§1.   History  of  discovery. 

§  2.   Rhythm  between  sexual  and  asexual  reproduction. 

§  3.  Alternation  between  sexual  and  degenerate  sexual  repro- 
duction. 

§  4.   Combination  of  both  these  alternations. 

§  5.  Alternation    of  juvenile    parthenogenetic    reproduction 
with  the  adult  sexual  process. 

§  6.  Alternation     of    parthenogenesis    and    ordinary   sexual 
reproduction. 

§  7.  Alternation  of  different  sexual  generations. 

§  8.   Occurrence  of  these  alternations  in  animals. 

§  9.   Occurrence  of  alternations  in  plants. 
§  10.  The  problem  of  heredity  in  alternating  generations. 
§  II.   Hints  as  to  the  rationale  of  alternation. 
§  12.  Origin  of  alternation  of  generations. 


BOOK  IV.— THEORY  OF  REPRODUCTION. 
CHAPTER  XVI. 

Growth  and  Reproduction   -----     219-231 

§  I.   Facts  of  growth. 

§  2.   Spencer's  analysis. 

§  3.   Cell-division. 

§  4.   Protoplasmic  restatement. 

§  5.   Antithesis  between  growth  and  reproduction. 

§  6.   The  contrast  in  the  individual — 

{a.)  In  distribution  of  organs. 

{/?.)  In  the  periods  of  life. 
§  7.   The  contrast  between  asexual  and  sexual  reproduction. 


CHAPTER  XVII. 

Theory  of  Reproduction — contimied  -  -  -     232-238 

§  I.   The  essential  fact  in  reproduction. 
§  2.   The  beginning  of  reproduction. 
§  3.   Cell-division. 

§  4.  Gradations  from  asexual  severance  to  liberation  of  sex- 
cells. 
§  5.  The  close  connection  between  reproduction  and  death. 
§  6.   Reproduction  as  influenced  by  the  environment. 
§  7.   General  conclusion. 


XVI  CONTENTS. 

CHAPTER  XVIII. 

PACE 

Special  Physiology  of  Sex  and  Reproduction       -  -     239-263 

§  I.  The  continuity  of  the  germ-plasma. 

§  2.  Sexual  maturity. 

§  3.  Menstruation. 

§  4.  Sexual  union. 

§  5.  Parturition. 

§  6.  Early  nutrition. 

§  7.  Lactation. 

§  8.  Other  secretions. 

§  9.  Incubation. 

§  10.  Nemesis  of  reproduction. 

§  II.  Love  and  death,  or  organic  immortality. 

CHAPTER  XIX. 

Psychological  and  Ethical  Aspects  -  -  264-282 

§  I.  Common  ground  between  animals  and  men. 

§  2.  The  love  of  mates. 

§  3.  Sexual  attraction. 

§  4.  Intellectual    and     emotional    differences    between    the 

sexes. 

§  5.  Love  for  offspring, 

§  6.  Criminal  habit  of  the  cuckoo. 

§  7.  Egoism  and  altruism. 

CHAPTER  XX. 

Laws  of  Multiplication         .  .  .  .  .    283-299 

§  I.  Rate  of  reproduction  and  rate  of  increase. 

§  2.  History  of  discussion, 

§  3.  Spencer's  analysis  ;  individuation  and  genesis. 

§  4.  Spencer's  application  to  man. 

§  5,  General  statement  of  the  population  question. 

§  6.  Sterility. 

CHAPTER  XXI. 

The  Reproductive  Factor  in  Evolution    -  -  -     300-315 

§  I.   General  history  of  evolution. 

§  2.  The  reproductive  factor  so  far  as  hitherto  recognised. 

§  3-   Suggested  lines  of  further  construction. 


BOOK     I. 


THE  SEXES  AND  SEXUAL  SELECTION. 


THE   EVOLUTION    OF   SEX. 


CHAPTER    I. 

The  Sexes  and  Sexual  Selection. 

THAT  all  higher  animals  are  represented  by  distinct  male 
and  female  forms,  is  one  of  the  most  patent  facts  of 
observation,  striking  enough  in  many  a  beast  and  bird  to  catch 
any  eye,  and  familiarly  expressed  in  not  a  few  popular  names 
which  contrast  the  two  sexes.  In  lower  animals,  the  contrast, 
and  indeed  the  separateness,  of  the  sexes  often  disappears;  yet 
even  naturalists  have  sometimes  mistaken  for  different  species, 
what  were  afterwards  recognised  to  be  but  the  male  and  female 
of  a  single  form. 

§  I.  Primary  and  Secondary  Chai'aders. — When  we  pass 
from  this  commonplace  of  observation  and  experience  to  inquire 
more  precisely  into  the  differences  between  the  sexes,  we  speedily 
recognise  that  these  are  of  very  different  degrees.  In  some  cases 
no  marked  differences  whatever  are  recognisable ;  thus  a  male 
star-fish  or  sea-urchin  looks  exactly  like  the  female,  and  a  care- 
ful examination  of  the  essential  reproductive  organs  is  requisite 
to  determine  whether  these  respectively  produce  male  elements 
or  eggs.  In  other  cases,  for  instance  in  most  reptiles,  no 
external  differences  are  at  all  striking,  but  the  aspect  of  the 
internal  organs,  both  essential  and  auxiliary  to  reproduction,  at 
once  settles  the  question.  In  a  great  number  of  cases,  again, 
the  sexes  resemble  one  another  closely,  but  each  has  certain 
minor  structural  features  at  once  decisive  as  to  its  respective 
maleness  or  femaleness.  Thus  in  the  males  there  are 
frequently  prominent  organs  used  in  sexual  union,  while  the 
peculiar  functions  of  the  females  are  indicated  in  the  special 
egg-laying    or   young-feeding   organs.       All    such    characters, 


THE    EVOLUTION    OF    SEX. 


directly  associated  with  the  essential  functions  of  the  sexes, 
are  included  under  the  title  oi primary  sexual  characters. 

Of  less  real  importance,  though  often  much  more  striking, 
are  the  numerous  distinctions  in  size,  colour,  skin,  skeleton,  and 


Male  and  Female  Bird  of  Paradise  {Paradiscn  i/iinor). — From 
Catalogue  of  Zoological  Museum,  Dresden. 

the  like,  which  often  signalise  either  sex.     These  are  termed 
secondary  sexual  characters ;  for  though  they  will  be  shown  in 


THE    SEXES    AND    SEXUAL    SELECTION.  5 

some  cases  at  least  to  be  truly  part  and  parcel  with  the  male  or 
female  constitution,  they  are  only  of  secondary  importance  in 
the  reproductive  process.  The  beard  of  man  and  the  mane 
of  the  lion,  the  antlers  of  stags  and  the  tusks  of  elephants,  the 
gorgeous  plumage  of  the  peacock  or  of  the  bird  of  paradise, 
are  familiar  examples  of  secondary  sexual  characters  in  males. 
Nor  are  the  females  lacking  in  special  characteristics,  which 
serve  as  indices  of  their  true  nature.  Large  size  is  one  of  the 
commonest  of  these ;  while  in  some  few^  cases  the  excellencies 
of  colour,  and  other  adornments,  are  possessed  by  the  females 
rather  than  by  their  mates. 

The  w^iole  subject  of  secondary  sexual  characters  has  found 
its  most  extensive  treatment  in  Darwin's  "  Descent  of  Man," 
and  to  that  work,  therefore,  the  more  so  as  its  limits  exceed 
those  of  the  present  volume,  the  reader  must  be  assumed  to 
make  reference.  All  that  can  be  here  attempted  is  an  illustra- 
tion, by  representative  cases,  of  the  main  differences  between 
the  sexes ;  from  which  we  shall  pass  to  Darwin's  interpretation, 
and,  after  a  fresh  survey,  to  the  explanation  by  which  we  propose 
to  supplement  his  theory. 

§  2.  Illustrations  from  Dariviii.  —  Among  invertebrates, 
prominent  secondary  sexual  characters  are  rarely  exhibited 
outside  the  great  division  of  jointed-footed  animals  or  arthro- 
pods. There,  however,  among  crustaceans  and  spiders,  but 
especially  among  insects,  beautiful  illustrations  abound.  Thus 
the  great  claws  of  crabs  are  frequently  much  larger  in  the 
males ;  and  male  spiders  often  differ  from  their  fiercely  coy 
mates,  in   smaller  size,  darker  colours,  and  sometimes  in  the 


Winged  Male  and  Wingless  Female  of  a  certain  Moth 
(JJrgyia  antiqua). — From  Leunis. 

power  of  producing  rasping  sounds.  Among  insects,  the  males 
are  frequently  distinguished  by  brighter  colours  attractively  dis- 
played, by  weapons  utilised  in  disposing  of  their  rivals,  and 
by  the  exclusive  possession  of  the  power  of  noisy  love-calling. 
Thus,  as  the  Greek  observed,  the  cicadas  "  live  happy,  having 
voiceless  wives."     Not  a  few  male  butterflies  are  pre-eminently 


6  THE    EVOLUTION    OF    SEX. 

more  brilliant  than  the  females ;  and  many  male  beetles  fight 
savagely  for  the  possession  of  their  mates. 

Passing  to  backboned  animals,  we  find  that  among  fishes 
the  males  are  frequently  distinguished  by  bright  colours  and 
ornamental  appendages,  as  well  as  by  structural  adaptations  for 
combat.  Thus  the  "gemmeous  dragonet  "  {Callionyniiis  lyrd)  is 
flushed  with  gorgeous  colour,  in  great  contrast  to  the  "  sordid  " 
female,  and  is  further  adorned  by  a  graceful  elongation  of  the 
dorsal  fin.  In  many  cases,  as  in  the  sea-scorpion  {^Cottus 
scorpius),  or  in  the  stickleback  {Gasterosteus\  it  is  only  at  the 
reproductive  period  that  the  males  are  thus  transformed, 
literally  putting  on  a  wedding-garment.  Every  one  knows,  on 
the  other  hand,  the  hooked  lower  jaw  of  the  male  salmon, 
which  comes  to  be  of  use  in  the  furious  charges  between  rivals; 
and  this  is  but  one  illustration  of  many  structures  utilised  in 
the  battle  for  mates.  In  regard  to  amphibians,  it  is  enough  to 
recall  the  notched  crests  and  lurid  colouring  of  our  male  newts, 
and  the  indefatigable  serenading  powers  of  male  frogs  and 
toads,  to  which  the  females  are  but  weakly  responsive.  Among 
reptiles,  differences  of  this  sort  are  comparatively  rare,  but  male 
snakes  have  often  more  strongly-pronounced  tints,  and  the 
scent-glands  become  more  active  during  the  breeding  season. 
In  this,  as  in  many  other  cases,  love  has  its  noisy  prayer  re- 
placed by  the  silent  appeal  of  fragrant  incense.  Among  lizards, 
the  males  are  often  more  brightly  decorated,  the  splendour  of 
their  colours  being  frequently  exaggerated  at  pairing  time. 
They  may  be  further  distinguished  by  crests  and  wattle-like 
pouches  ;  while  horns,  probably  used  in  fighting,  are  borne  by 
some  male  chamseleons. 

It  is  among  birds,  however,  that  the  organic  apparatus  of 
courtship  is  most  elaborate.  The  males  very  generally  excel  in 
brighter  colours  and  ornaments.  Beautiful  plumes,  elongated 
feathery  tresses,  brightly-coloured  combs  and  wattles,  top-knots 
and  curious  markings,  occur  with  marvellous  richness  of  variety. 
These  are  frequently  displayed  by  their  proud  possessors  before 
the  eyes  of  their  desired  mates,  with  mingled  emotions  of  eager 
love  and  pompous  vanity;  or  it  may  be  to  the  subtler  charms  of 
music  that  the  wooers  mainly  trust.  During  the  breeding 
season,  the  males  are  jealously  excited  and  pugnacious,  while 
some  have  special  weapons  for  dealing  directly  with  their  rivals. 
The  differences  between  the  magnificent  male  l)irds  of  paradise 
and  their  sober  mates,  between  the  peacock  with  his  hundred 


THE    SEXES    AND    SEXUAL    SELECTION.  7 

eyes  and  the  plain  peahen,  between  the  musical  powers  of 
male  and  female  songsters,  are  very  familiar  facts.  Or  again, 
the  combs  and  "gills"  of  cocks,  the  "wattles"  of  turkey-cocks, 
the  immense  top-knot  of  the  male  umbrella-bird  {Cephalopterus 
ornatiis),  the  throat-pouch  of  the  bustard, — illustrate  another 


Male  and  Female  Blackcocks. 

series  of  secondary  sexual  characters.  The  spurs  of  cocks  and 
allied  birds  are  the  most  familiar  illustrations  of  weapons  used 
by  the  males  in  fighting  with  rivals.  As  in  other  animals,  it  is 
important  to  notice  that  male  birds  often  acquire  their  special 
secondary  characters,  such  as  colour,  markings,  and  special 
forms  of  feathers,  only  as  they  approach  sexual  maturity,  and 
sometimes  retain  them  in  all  their  glory  only  during  the 
breeding  season. 

Among  mammals,  which  stand  in  so  many  ways  in  marked 
contrast  to  birds,  the  law  of  battle  much  more  than  the  power 
of  charming  decides  the  problem  of  courtship.  Thus  most 
of  the  striking  secondary  characters  of  male  mammals  are 
weapons.  Yet  there  are  crests  and  tufts  of  hair,  and  other 
acknowledgments  of  the  beauty  test,  while  the  incense  of 
odoriferous  glands  is  a  very  frequent  means  of  sexual  attrac- 


THE    EVOLUTION    OF    SEX. 


tion.  The  colours  too  of  the  males  are  often  more  sharply 
contrasted,  and  there  are  minor  differences,  in  voice  and  the 
like,  which  cannot  be  ignored.  Of  weapons,  the  larger  canine 
teeth  of  many  male  animals,  such  as  boars  ;  the  special  tusks 
of,  for  instance,  the  elephant  and  narwhal ;  the  antlers  of  stags, 


The  development  of  antlers  in  the  successive  years  of  a 
s'as^s  life,  or  in  the  general  history  of  stags.  —  From 
Cams  Sterne. 

all  but  exclusively  restricted  to  the  combative  sex ;  the  horns 
of  antelopes,  goats  and  sheep,  oxen  and  the  like, — which  at  least 
predominate  in  the  males, — are  well-known  illustrations.  The 
manes  of  male  lions,  bisons,  and  baboons ;  the  beards  of 
certain  goats ;  the  crests  along  the  backs  of  some  antelopes ; 
the  dewlaps  of  bulls, — illustrate  another  set  of  secondary 
characters.  The  odoriferous  glands  of  many  mammals  are  more 
developed  in  the  males,  and  become  specially  functional  during 
the  breeding  season.  This  is  well  illustrated  in  the  case  of 
goats,  deer,  shrew-mice,  elephants.  The  differences  in  colour 
are  slight  compared  with  those  seen  between  the  sexes  in  birds, 
but  in  not  a  few  orders  the  distinction  is  marked  enough,  males 
being,  in  the  great  majority  of  cases,  the  more  strongly  and 
brilliantly  coloured.  Among  monkeys  the  difference  in  colour 
in  the  bare  regions,  and  the  subtler  decorations  in  the  arrange- 
ment of  the  hair  on  the  face,  are  often  very  conspicuous. 

§  3.  Darwin's  Explanation  —  Sexual  Selection.  —  Darwin 
started  from  the  occurrence  of  such  variations,  in  structure  and 
habit,  as  might  be  useful  either  for  attraction  between  the  sexes 
or  in  the  direct  contests  of  rival  males.  The  possessors  of 
these  variations  succeeded  l)etter  than  their  neighbours  in  the 
art  of  courtship ;  the  factors  which  constituted  success  were 
transmitted  to  the  offspring ;  and,  gradually,  the  variations  were 


THE    SEXES    AND    SEXUAL    SELECTION.  9 

established  and  enhanced  as  secondary  sexual  characters  of  the 
species.  The  process  by  which  the  possessors  of  the  fortunate 
excellencies  of  beauty  and  strength  outbid  or  overcome  their 
less  endowed  competitors,  he  termed  "sexual  selection."  It  is 
only  fiiiY,  however,  to  state  Mr  Darwin's  case  by  direct 
quotation. 

Sexual  selection  "  depends  on  the  advantage  which  certain 
individuals  have  over  others  of  the  same  sex  and  species  solely 
in  respect  of  reproduction."  ...  In  cases  w^here  "  the  males 
have  acquired  their  present  structure,  not  from  being  better 
fitted  to  survive  in  the  struggle  for  existence,  but  from  having 
gained  an  advantage  over  other  males,  and  from  having  trans- 
mitted this  advantage  to  their  male  offspring  alone,  sexual 
selection  must  have  come  into  action."  ...  "A  slight  degree 
of  variability,  leading  to  some  advantage,  however  slight,  in 
reiterated  deadly  contests,  would  suffice  for  the  work  of  sexual 
selection."  ...  So  too,  on  the  other  hand,  the  females  "  have, 
b//  a  long  selection  of  the  more  attractive  males,  added  to  their 
'eauty  or  other  attractive  qualities."  ..."  If  any  man  can  in 
/c\  short  time  give  elegant  carriage  and  beauty  to  his  bantams, 
according  to  his  standard  of  beauty,  I  can  see  no  reason  to 
doubt  that  female  birds,  by  selecting  during  thousands  of 
generations  the  most  melodious  or  beautiful  males,  according 
to  their  standard  of  beauty,  might  produce  a  marked  effect." 
.  .  .  "To  sum  up  on  the  means  through  which,  as  far  as  we 
can  judge,  sexual  selection  has  led  to  the  development  of 
secondary  sexual  characters.  It  has  been  shown  that  the 
largest  number  of  vigorous  offspring  will  be  reared  from  the 
pairing  of  the  strongest  and  best-armed  males,  victorious  in 
contests  over  other  males,  with  the  most  vigorous  and  best- 
nourished  females,  which  are  the  first  to  breed  in  the  spring. 
If  such  females  select  the  more  attractive,  and  at  the  same 
time  vigorous  males,  they  will  rear  a  larger  number  of  offspring 
than  the  retarded  females,  which  must  pair  with  the  less 
viirorous  and  less  attractive  males.     So  it  will  be  if  the  more 

O 

vigorous  males  select  the  more  attractive,  and  at  the  same  time 
healthy  and  vigorous  females  ;  and  this  will  especially  hold 
good  if  the  male  defends  the  female,  and  aids  in  providing 
food  for  the  young.  The  advantage  thus  gained  by  the  more 
vigorous  pairs  in  rearing  a  larger  number  of  offspring,  has 
apparently  sufficed  to  render  sexual  selection  efficient." 
Another  sentence  from  Darwin's  first  statement  of  his  position 


lO  THE    EVOLUTION    OF    SEX. 

must,  however,  be  added.  "  I  would  not  wish,"  he  says  in 
the  "Origin  of  Species,"  "to  attribute  all  such  sexual  differences 
to  this  agency  ;  for  we  see  peculiarities  arising  and  becoming 
attached  to  the  male  sex  in  our  domestic  animals,  which  we 
cannot  believe  to  be  either  useful  to  the  males  in  battle  or 
attractive  to  the  females."  Had  Darwin  seen  another  inter- 
pretation of  the  facts,  he  would  thus  doubtless  have  given  it 
frank  recognition. 

§  4.  Criticisms  of  Darivift's  Expla?mtion. — The  above 
explanation  may  be  summed  up  in  a  single  sentence, — a  casual 
variation,  advantageous  to  its  possessor  (usually  a  male)  in 
courtship  and  reproduction,  becomes  established  and  perfected 
by  the  success  it  entails.  Sexual  selection  is  thus  only  a 
special  case  of  the  more  general  process  of  natural  selection, 
with  this  difference,  that  the  female  for  the  most  part  takes  the 
place  of  the  general  environment  in  the  picking  and  choosing 
which  is  supposed  to  work  out  the  perfection  of  the  species. 

The  more  serious  objections  which  have  been  hitherto  urged 
against  this  hypothesis,  apart  altogether  from  criticism  of  special 
cases,  may  be  grouped  in  four  grades : — ( i )  Some,  who  allow  great 
importance  to  both  natural  and  sexual  selection,  are  dissatisfied 
with  the  adequacy  of  Darwin's  analysis,  and  seek  some  deeper 
basis  for  the  variations  so  largely  confined  to  the  male  sex. 
The  position  occupied  by  Brooks  will  be  sketched  below.  (2) 
Others  would  explain  the  facts  on  the  more  general  theory  of 
natural  selection,  allowing  comparatively  little  import  to  the 
alleged  sexual  selection  exercised  by  the  female.  Wallace  has 
on  this  basis  criticised  Darwin's  theory.  (3)  Different  from 
either  of  the  above  is  the  position  occupied  by  St  George 
Mivart,  who  attaches  comparatively  little  importance  to  either 
natural  or  sexual  selection.  (4)  We  have  to  recognise  contri- 
butions, such  as  those  of  Mantegazza,  which  suggest  the  organic 
or  constitutional  origin  of  the  variations  in  question.  It  is  this 
constructive  rather  than  destructive  line  of  criticism  which  we 
shall  ourselves  seek  to  develop. 

{a)  Wallace's  Objection.— \\.  is  more  convenient  to  begin  with 
Wallace's  criticism,  which  precedes  that  of  Brooks's  in  chrono- 
logical order.  This  is  the  more  helpful  in  clearing  the  ground, 
since  the  two  theories  of  Wallace  and  Darwin  are  strikingly 
and,  at  first  sight,  irreconcilably  opposed.  According  to  Darwin, 
the  gayness  of  male  birds  is  due  to  selection  on  the  i)art  of  the 
females ;  according  to  Wallace,  the  soberness  of  female  birds  is 


THE    SEXES    AND    SEXUAL    SELECTION.  II 

due  to  natural  selection,  which  has  eliminated  those  which 
persisted  to  the  death  in  being  gay.  He  points  out  that 
conspicuousness  during  incubation  would  be  dangerous  and 
fatal ;  the  more  conspicuous  have,  he  thinks,  been  picked  off 
their  nests  by  hawks,  foxes,  and  the  like,  and  hence  only  the 
sober-coloured  females  now  remain.  Darwin  starts  from 
inconspicuous  forms,  and  derives  gorgeous  males  by  sexual 
selection ;  Wallace  starts  from  conspicuous  forms,  and  derives 
the  sober  females  by  natural  selection ;  the  former  trusts  to  the 
preservation  of  beauty,  the  latter  to  its  extinction.  In  1773, 
the  Hon.  Daines  Barrington,  a  naturalist  still  remembered  as 
the  correspondent  of  Gilbert  White,  suggested  that  singing- 
birds  were  small,  and  hen-birds  mute  for  safety's  sake.  This 
suggestion  Wallace  has  repeated  and  elaborated  in  reference 
especially  to  birds  and  insects.  The  female  butterfly,  exposed 
to  danger  during  egg-laying,  is  frequently  dull  and  inconspicuous 
compared  with  her  mate.  The  original  brightness  has  been 
forfeited  by  the  sex  as  a  ransom  for  life.  Female  birds  in  open 
nests  are  similarly,  in  many  cases,  coloured  like  their  sur- 
roundings ;  while  in  those  of  birds  where  the  nests  are  domed 
or  covered,  the  plumage  is  gay  in  both  sexes.  At  the  same 
time,  Wallace  allows  original  importance  to  sexual  selection  on 
both  sides  in  evolving  bright  colours  and  the  like.  We  need 
not  repeat  Darwin's  reply  to  Wallace's  objections,  as  the  reader 
will  at  once  recognise  considerable  force  in  each  position."^ 

(b)  Brooks  has  called  attention  to  the  sexual  differences  in 
lizards,  where  the  females  do  not  incubate  ;  or  in  fishes,  where 
the  females  are  even  less  exposed  to  danger  than  the  males ; 
or  in  domesticated  birds,  where,  though  all  danger  is  removed, 
the  males  are  still  the  more  conspicuous  and  diversified  sex. 

*  Since  the  above  was  written,  Mr  Wallace's  book  on  "Darwinism"  has 
been  published,  in  which  the  author  proceeds  yet  further  in  his  destructive 
criticism  of  Darwin's  sexual  selection.  The  phenomena  of  male  ornament 
are  discussed,  and  summed  up  as  being  "  due  to  the  general  laws  of  growth 
and  development,"  and  such  that  it  is  "unnecessary  to  call  to  our  aid  so  hypo- 
thetical a  cause  as  the  cumulative  action  of  female  preference."  Or  again, 
"  if  ornament  is  the  natural  product  and  direct  outcome  of  superabundant 
health  and  vigour,  then  no  other  mode  of  selection  is  needed  to  account 
for  the  presence  of  such  ornament."  These  conclusions  are  not  only 
important  in  relation  to  Darwin's  theory,  but  obviously  open  up  the  pos- 
sibility of  interpreting  not  only  these  as  the  "natural  product  and  direct 
outcome  of  constitutional  conditions  "  (j-<?(?chap.  xxi.)  but  many  other  features 
also.  This  consideration,  however,  is  fraught  with  serious  consequences  to 
Mr  Wallace's  main  thesis. 


12  THE    EVOLUTION    OF    SEX. 

"  The  fact  too  that  many  structures,  which  are  not  at  all  con- 
spicuous, are  confined,  like  gay  plumage,  to  male  birds,  also 
indicates  the  existence  of  an  explanation  more  fundamental 
than  the  one  proposed  by  Wallace,  and  the  latter  explanation 
gives  no  reason  why  the  females  of  allied  species  should  often 
be  exactly  alike  when  the  males  are  very  different."  To  the 
explanation  which  Brooks  proposes  we  must  therefore  pass. 

According  to  Darwin,  Brooks  says,  the  greater  modification 
of  the  males  is  due  to  their  struggling  with  rivals,  and  to  their 
selection  by  the  females,  but  "  I  do  not  believe  that  this  goes 
to  the  root  of  the  matter."  The  study  of  domesticated  pigeons, 
for  instance,  shows  that  "something  within  the  animal 
determines  that  the  male  should  lead  and  the  female  follow  in 
the  evolution  of  new  breeds.  The  same  is  true  in  other 
domesticated  animals,  where,  from  the  nature  of  the  circum- 
stances, it  is  inadmissible  to  explain  this  with  Darwin,  by 
supposing  that  the  male  is  more  exposed  than  the  female  to 
the  action  of  selection,  whether  natural  or  sexual.  Darwin 
concludes,  indeed,  that  the  male  is  more  variable  than  the 
female,  but  he  gives  no  satisfactory  reason  why  female 
variations  should  be  less  apt  than  male  variations  to  become 
hereditary,  or,  in  other  words,  why  the  right  of  entail  is  so  much 
restricted  to  the  male  sex.  Darwin  merely  attributes  this  to 
the  greater  eagerness  of  the  males,  which  "  in  almost  all 
animals  have  stronger  passions  than  the  females."  The  theory 
which  Brooks  maintains,  is  bound  up  with  an  hypothesis  of 
heredity  differing  considerably  from  that  held  by  Darwin.  He 
supposes  that  the  cells  of  the  body  give  off  gemmules,  chiefly 
during  change  of  function  or  of  environment,  and  that  "  the 
male  reproductive  cell  has  gradually  acquired,  as  its  especial  and 
distinctive  function,  a  peculiar  power  to  gather  and  store  up 
these  gemmules."  The  female  reproductive  cells  keep  up  the 
general  constancy  of  the  species,  the  male  cells  transmit 
variations.  "  A  division  of  physiological  labour  has  arisen 
during  the  evolution  of  life,  and  the  functions  of  the  repro- 
ductive elements  have  become  specialised  in  different  direc- 
tions." "  The  male  cell  became  adapted  for  storing  up 
gemmules  "  (the  results  of  variations  in  the  body),  "  and  at  the 
same  time  gradually  lost  its  unnecessary  and  useless  jDOwer  to 
transmit  hereditary  characteristics."  "  We  thus  look  to  the 
cells  of  the  male  body  for  the  origin  of  most  of  the  variations 
through  which  the  species  has  attained  to  its  present  organisa- 


THE    SEXES    AND    SEXUAL    SELECTION.  13 

tion."  The  males  are  the  more  variable,  but  more  than  that, 
their  variations  are  much  more  likely  to  be  transmitted.  •'  We 
are  thus  able  to  understand  the  great  difference  in  the  males 
of  allied  species,  the  difference  between  the  adult  male  and  the 
female  or  young,  and  the  great  diversity  and  variability  of 
secondary  male  characters ;  and  we  should  expect  to  find,  what 
actually  is  the  case,  that  among  the  higher  animals,  when  the 
sexes  have  long  been  separated,  the  males  are  more  variable 
than  the  females."  The  contrast  between  Darwin  and  Brooks 
may  now  be  summed  up  again  in  a  sentence.  Darwin  says, 
the  males  are  more  diversified  and  richer  in  secondary  sexual 
characters,  chiefly  because  of  the  sexual  selection  exercised 
alike  in  courtship  and  in  battle.  Brooks  admits  sexual 
selection,  but  finds  an  explanation  of  the  greater  diversity  of 
the  males  in  his  theory  that  it  is  the  peculiar  function  of  the 
male  elements  to  transmit  variations,  as  opposed  to  the  constant 
tradition  of  structure  kept  up  by  the  egg-cells  or  ova.  In 
other  words,  the  females  may  choose,  yet  the  males  lead  ;  nay 
more,  they  must  lead,  for  male  variations  have  by  hypothesis 
most  likelihood  of  being  transmitted. 

Full  consideration  of  this  hypothesis  would  involve  much 
discussion  of  the  problems  of  inheritance,  which  will  form  the 
subject  of  a  forthcoming  volume ;  but  the  general  conclusion 
of  the  naturally  greater  variability  of  the  males,  will  be  stated  in 
a  different  light  towards  the  close  of  the  following  chapter.  It 
will  there  be  shown  that  the  "something  within  the  animal," 
which  determines  the  preponderance  of  male  variability,  may 
be  stated  in  simpler  terms  than  are  involved  in  Brooks's  theory 
of  heredity.  To  refer  preponderant  male  variability  back  to  a 
power,  ascribed  to  the  male  reproductive  cells,  of  collecting  and 
storing  up  assumed  gem  mules,  is  at  best  but  a  half-way  analysis. 

Both  the  above  critics  are  at  one  with  Darwin  on  essential 
points.  Though  Wallace  would  explain  by  natural  selection 
what  Darwin  explained  by  sexual  selection,  he  does  not  deny 
the  importance  of  the  latter  in  many  cases.  Brooks,  again, 
emphasises  a  deeper  factor,  without  doubting  the  general  truth 
of  Darwin's  account  of  the  process.  Different  from  both 
these  positions  is  that  (c)  occupied  by  St  George  Mivart,  who 
looks  for  some  deeper  reason  than  either  Darwin  or  Wallace 
suggest.  The  entire  theory  of  sexual  selection  appears  to  him 
an  unverified  hypothesis,  only  acquiring  plausibility  when  sup- 
ported by  quite  a  series  of  subsidiary  suppositions.     He  submits 


14  THE    EVOLUTION    OF    SEX. 

a  number  of  detailed  criticisms;  but  his  chief  contention  is,  that 
the  beauty  of  males,  and  other  secondary  sexual  characters,  are 
not  the  indirect  results  of  a  long  process  of  external  selection, 
but  the  direct  expressions  of  an  internal  force. 

The  vague  suggestions  of  Mantegazza  and  others  are  only 
of  importance  as  indications  of  progress  towards  a  fundamental 
explanation.  An  obvious  objection  to  the  theory  of  sexual 
selection,  that  has  been  urged  by  many,  is  that,  while  it  may 
in  part  account  for  the  persistence  and  progress  of  secondary 
characters  after  they  attained  a  certain  degree  of  development, 
it  does  not  account  for  their  preservation  when  weak  or  incon- 
spicuous ;  in  short,  the  theory  may  account  for  the  perfecting, 
but  not  for  the  origin  of  the  characters.  It  may  be  enough  to 
account  for  the  length  and  the  trimmings  of  the  living  garment, 
but  what  we  wish  to  know  is  the  secret  of  the  loom.  Darwin's 
account  of  the  evolution  of  the  eyes  on  the  feathers  of  the  Argus 
pheasant  is  indeed  ingenious  and  interesting ;  but,  whatever  its 
probability,  it  is  more  important  to  ask  what  the  predominant 
brightness  of  males  means  as  a  general  fact  in  physiology.  It 
is  of  interest,  then,  to  notice  the  hints  thrown  out  by  Mante- 
gazza, Wallace,  and  others,  directly  associating  decorativeness 
with  superfluous  reproductive  material,  and  the  putting  on  of 
wedding-robes  with  the  general  excitement  of  the  sexually 
mature  organism.  From  this  record  of  the  discussion,  it  is 
time  however  to  turn  to  a  more  constructive  mode  of  treat- 
ment. 


THE    SEXES    AND    SEXUAL    SELECTION.  15 


SUMMARY. 

I,  2.  The  existence  of  male  and  female  animals  is  a  commonplace  of 
observation.  They  differ  in  primary  and  in  secondary  sexual  characters, 
of  which  illustrations  are  given,  chiefly  from  Darwin. 

3.  Darwin's  hypothesis  of  sexual  selection  assumes  the  preservation  and 
perfection  of  variations,  advantageous  in  courtship  or  in  battles  with  rivals. 

4.  \Yallace  maintains  that  the  females  have  been  protectively  retarded 
by  natural  selection  ;  Brooks,  that  the  males  predominate  in  power  of 
transmitting  variations,  and  are  therefore  more  divergent;  while  Mivart 
demands  a  deeper  analysis  than  is  afforded  by  either  sexual  or  natural  selec- 
tion,— such  a  physiological  rationale  being  hinted  at. 


LITERATURE. 

Brooks  (W.  K.)— The  Law  of  Heredity  :  A  Study  of  the  Cause  of  Varia- 
tion and  the  Origin  of  Living  Organisms.     Baltimore,  1S83. 

Darwin  (C.) — On  the  Origin  of  Species  by  Means  of  Natural  Selection  ; 
or.  The  Preservation  of  Favoured  Races  in  the  Struggle  for  Life. 
London,  1859. 

The  Descent  of  Man,  and  Selection  in  Relation  to  Sex.     London, 

1871. 

Mivart  (St  George) — Lessons  from  Nature.     London,  1876. 

Wallace  (A.  R.) — Contributions  to  the  Theory  of  Natural  Selection. 
London,  1871. 

Darwinism  :  An  Exposition  of  the  Theory  of  Natural   Selection, 

with  Some  of  its  Applications.     London,  1889. 


CHAPTER    IL 

The  Sexes,  and  Criticism  of  Sexual  Selection. 

§  I.  To  gain  a  firmer  and  broader  foundation  on  which  to 
base  a  theory  of  the  differences  between  the  sexes,  it  is  ne- 
cessary to  take  another  review  of  the  facts  of  the  case.  Instead 
of  considering  the  differences  as  they  are  expressed  in  the 
successive  classes  of  animals,  it  will  be  more  convenient  to 
arrange  them  for  themselves,  according  as  they  affect  habit, 
size,  length  of  life,  and  the  like.  The  review  must  again  be 
merely  representative,  without  any  attempt  at  completeness. 


Male  and  Female  Coccus  Insects,  a,  part  of  a  cactus 
plant  with  the  excrescences  due  to  coccus  insects ; 
?',  male  ;  c,  female. 

^  2.  General  Habit. — Let  us  begin  with  an  extreme  yet 
well-known  case.  The  female  cochineal  insect,  laden  with 
reserve  products  in  the  form  of  the  well-known  pigment,  spends 


THE    SEXES,    AND    CRITICISM    OF    SEXUAL    SELECTION.         I  7 

much  of  its  life  like  a  mere  quiescent  gall  on  the  cactus  plant. 
The  male,  on  the  other  hand,  in  his  adult  state  is  agile, 
restless,  and  short-lived.  Now  this  is  no  mere  curiosity  of  the 
entomologist,  but  in  reality  a  vivid  emblem  of  what  is  an  aver- 
age truth  throughout  the  world  of  animals — the  preponderating 
passivity  of  the  females,  the  predominant  activity  of  the  males. 
These  coccus  insects  are  the  martyrs  of  their  respective  sexes. 
Take  another  illustration,  again  somewhat  extreme.  There  is 
a  troublesome  threadworm  {^Heterodcra  schachtii)  infesting  the 
turnip  plant,  which  parallels  in  more  ways  than  one  the  contrast 
of  the  coccus  insects.  The  adult  male  is  agile,  and  like  many 
another  threadworm  ;  the  adult  female,  however,  is  quiescent, 
and  bloated  like  a  drawn-out  lemon.  It  may  be  asked,  how- 
ever, is  not  this  merely  the  natural  nemesis 
of  parasitism?  The  life-history  answers 
this  objection.  The  two  sexes  are  at  first 
alike, — agile,  and  resembling  most  thread- 
worms ;  they  become  parasitic,  and  lose 
both  activity  and  nematode  form  ;  but  the 
interesting  fact  is  further,  that  the  male 
recovers  himself,  while  the  female  remains  a 
victim.  In  other  insect  and  worm  types 
the  same  story,  in  less  accented  characters, 
may  be  distinctly  read.  In  many  crusta- 
ceans, again,  the  femalesonly  are  parasitic; 
and  while  this  is  in  part  explained  by  their 
habit  of  seeking  shelter  for  egg-laying  pur- 
poses, it  also  expresses  the  constitutional 
bias  of  the  sex.  The  insect  order  of  bee 
parasites  {Strepsipterd)  is  remarkable  for  the 
completely  passive  and  even  larval  character 
of  the  blind  parasitic  females,  while  the 
adult  males  are  free,  winged,  and  short-lived. 

Throughout    the  class    of    insects    there   are     Female  Clwndracanthus,  a 
•11       i^       i  ■  r     ii  „„n  ^.,^^  parasitic       Crustacean, 

numerous    illustrations    oi    the  excellence         ^^^h  pigmy  male  («) 
of  the    males   over  the  females,   alike  in        attached^just  above  the 

origin  of   the  long  egg- 

muscular   power   and    sensory    acuteness.         sacs  {b)  of  the  female. 
The  diverse  series  of  efforts  by  which  the        -^''^'^  <^'^"^- 
males   of  so   many    different    animals,   from   cicadas   to   birds, 
sustain  the  love-chorus,  affords  another  set  of  illustrations  of 
pre-eminent  masculine  activity. 

Without    multiplying   instances,    a   review   of    the    animal 

B 


THE    EVOLUTION    OF    SEX. 


kingdom,  or  a  perusal  of  Darwin's  pages,  will  amply  confirm  the 
conclusion  that  on  an  average  the  females  incline  to  passivity, 
the  males  to  activity.  In  higher  animals,  it  is  true  that  the 
contrast  shows  rather  in  many  little  ways  than  in  any  one 
striking  difference  of  habit,  but  even  in  the  human  species  the 
contrast  is  recognised.  Every  one  will  admit  that  strenuous 
spasmodic  bursts  of  activity  characterise  men,  especially  in 
youth,  and  among  the  less  civilised  races  ;  while  patient  con- 
tinuance, with  less  violent  expenditure  of  energy,  is  as  generally 
associated  with  the  work  of  women. 


Both  sexes  of  a  Flea — the  Jigger  or  Ch\goe(Sarco/sj>//a  />i'nc'fra/is); 
female  much  swollen  with  eggs. — From  Leuckart. 


the 


For  completeness  of  argument,  two  other  facts,  which  will 
afterwards  claim  full  discussion,  may  here  be  simply  mentioned. 
(a)  At  the  very  threshold  of  sex-difference,  we  hnd  that  a  little 
active  cell  or  spore,  unable  to  develop  of  itself,  unites  in 
fatigue  with  a  larger  more  quiescent  individual.  Here,  at  the 
very  first,  is  the  contrast  between  male  and  female.  (/>')  The 
same  antithesis  is  seen,  when  we  contrast,  as  we  shall  afterwards 


THE    SEXES,    AND    CRITICISM    OF    SEXUAL    SELECTION.         1 9 

do  in  detail,  the  actively  motile,  minute,  male  element  of  most 
animals  and  many  plants,  with  the  larger  passively  cjuiescent 
female-cell  or  ovum. 

It  is  possible  that  the  reader  may  urge  as  a  difficulty  against 
the  above  contrast  the  exceedingly  familiar  case  of  the  male 
bees  or  "drones."  It  must  be  frankly  allowed  that  exceptions 
do  indeed  occur,  though  usually  in  conditions  which  afford  a 
key  to  the  abnormality.  Thus  it  will  be  allowed  that  the 
"drones"  are  in  a  peculiar  position  as  male  members  of  a  very 
complex  society,  in  which  what  is  practically  a  third  sex  is 
represented  by  the  great  body  of  "workers."  They  are  no 
more  fair  examples  of  the  natural  average  of  males,  than  the 
hard-driven  wives  of  the  lazy  Kaffir  are  of  the  normal  functions 
of  women.  Nor  is  the  exception  even  here  a  real  one,  for  the 
drone,  although  passive  as  compared  with  the  unsexed  workers, 
is  active  when  compared  with  the  extraordinarily  passive  queen. 

To  the  above  contrast  of  general  habit,  two  other  items 
may  be  added,  on  which  accurate  observation  is  still  unfortun- 
ately very  restricted.  In  some  cases  the  body  temperature, 
which  is  an  index  to  the  pitch  of  the  life,  is  distinctly  lower  in  the 
females,  as  has  been  noted  in  cases  so  widely  separate  as  the 
human  species,  insects,  and  plants.  In  many  cases,  further- 
more, the  longevity  of  the  females  is  much  greater.  Such  a 
fact  as  that  women  pay  lower  insurance  premiums  than  do 
men,  is  often  poi)ularly  accounted  for  by  their  greater  immunity 
from  accident ;  but  the  greater  normal  longevity  on  which  the 
actuary  calculates,  has,  as  we  begin  to  see,  a  far  deeper  and 
constitutional  explanation. 

!$  3.  Size. — Among  the  higher  animals,  there  are  curious 
alternations  in  the  preponderance  of  one  sex  over  another  in 
size.  Thus  among  mammals  and  birds  the  males  are  in  most 
cases  the  larger ;  the  same  is  true  of  lizards ;  but  in  snakes  the 
females  preponderate.  In  fishes,  the  males  are  on  an  average 
smaller,  sometimes  very  markedly  so,  even  to  the  extent  of  not 
being  half  as  large  as  tlieir  mates.  Below  the  line,  among 
backboneless  animals,  there  is  much  greater  constancy  of 
predominance  in  favour  of  the  females.  Thus  among  insects, 
the  more  active  males  are  generally  smaller,  and  often  very 
markedly ;  of  spiders  the  same  is  true,  and  the  males  being 
often  very  diminutive  are  forced  to  task  their  agility  to  the 
utmost  in  making  advances  to  their  unamiable  mates.  So 
again,  crustacean  males  are  often  smaller  than  the  females  ;  and 


20 


THE    EVOLUTION    OF    SEX. 


in  many  parasitic  species,  what  have  been  well  called  "  pigmy  " 
males  illustrate  the  contrast  in  an  almost  ludicrous  degree. 

Two  cases  from  aberrant  worm  types  exhibit  very  vividly 
this  same  antithesis  of  size.  Among  the  common  rotifers,  the 
males  are  almost  always  very  different  from  the  females,  and 
much  smaller.  Sometimes  they  seem  to  have  dwindled  out  of 
existence  altogether,  for  only  the  females  are  known.  In  other 
cases,  though  present,  they  entirely  fail  to  accomplish  their 
proper  function  of  fertilisation,  and,  as  parthenogenesis  obtains, 
are  not  only  minute,  but  useless.      In  a  curious  green  marine 


Relative  sizes  of  a  male  and  female  Rotifer  (^1 1 ydatina  scuta). 
—  From  Leuiiis. 

worm,  Bonellia,  the  male  remains  like  a  remote  ancestor  of  the 
female.  It  lives  parasitically  on  or  within  the  latter,  and  is 
microscopic  in  size,  measuring  in  fact  only  al)out  one  hundredth 
part  of  the  length  of  its  host  and  mate.  Somewhat  similar  to 
the  case  of  bonellia  is  that  of  a  viviparous  coccus  insect  {Lecauiuni 
hespe7'idujii)^  where  the  males  are  very  degenerate,  small,  blind, 
and  wingless.  In  spite  of  this  condition,  we  should  indeed 
think  because  of  it,  they  are  very  male,  for  even  the  larvce, 
while  still  within  the  mother,  have  been  shown  to  contain  fully- 
develo})ed  s])ermat()zoa. 

It  would  be  unfair  to  argue  from  such  an  extreme  case  as 
that   of  Bonellia  alone,  but  there   is  no  doubt  that  up  to   the 


THE    SEXES,    AND    CRITICISM    OF    SEXUAL    SELECTION.         2  1 


level  of  amphibians  at  least  the  females  are  generally  the  larger. 
'J'his  then  must  be  taken  in  connection  with  the  conclusion  of 
the  previous  paragraph.  A  sluggish  conservative  habit  of  body 
tends  to  an  increase  of  size  ;  lavish  expenditure  of  energy  keeps 
down  the  accumulation  of  storage.  Corroborative  evidence 
will  be  afterwards  forthcoming,  as  we  contrast  (a)  the  large  and 
small  spores  which  mark  the  beginnings  of  sex  differences,  or 
(fi)  the  relatively  large  female  cell  or  egg  with  the  microscopic 
male  cell  or  spermatozoon. 


Figure  of  the  Temale  Ronellia  (from  Atlas  of  Naples  Aquarium), 
with  its  parasitic  pigmy  male  enlarged. 

Apparent  exceptions  occur,  it  is  true,  among  the  higher 
animals.  In  birds  and  mammals  the  males  are  usually  rather 
larger  than  the  females.  This  difference  consists  especially  in 
larger  bones  and  muscles.  The  apparent  exception  is  in  part 
the  natural  result  of  the  increased  stress  of  external  activities 
which  are  thrown  upon  the  shoulders  of  the  males  when  their 
mates  are  incapacitated  by  incubation  and  pregnancy.  Further- 
more, we   must  recognise  the   strengthening  influence   of  the 


22  THE    EVOLUTION    OF    SEX. 

combats  between  males,  and  the  effect  produced  on  the 
accumulative  constitution  of  the  females  by  the  increased 
maternal  sacrifice  characteristic  of  the  highest  animals. 

§  4.  Other  Cha7'acters. — A\'hile  it  is  easy  to  point  to  the 
general  physiological  import  of  large  size  and  the  reverse, 
physiology  is  not  yet  far  enough  advanced  to  afford  firm  foot- 
hold in  dealing  with  the  details  of  secondary  sexual  characters. 
It  is  only  possible  to  point  out  the  path  which  will  eventually 
lead  us  to  their  complete  rationale.  This  path  will  appear  less 
vague  if  reverted  to  after  some  of  the  succeeding  chapters  have 
been  grasped.  The  point  of  view  is  simple  enough.  The 
agility  of  males  is  not  a  special  adaptation  to  enable  that  sex 
to  exercise  its  functions  with  relation  to  the  other,  but  is  a 
natural  characteristic  of  the  constitutional  activity  of  maleness  ; 
and  the  small  size  of  many  male  fishes  is  not  an  advantage  at  all, 
but  simply  again  the  result  of  the  contrast  between  the  more 
vegetative  growth  of  the  female  and  the  costly  activity  of  the 
male.  So,  brilliancy  of  colour,  exuberance  of  hair  and  feathers, 
activity  of  scent-glands,  and  even  the  development  of  weapons, 
are  not,  and  cannot  be  (except  teleologically),  explained  by 
sexual  selection,  but  in  origin  and  continued  development  are 
outcrops  of  a  male  as  opposed  to  a  female  constitution.  To 
sum  up  the  position  in  a  paradox,  all  secondary  sexual 
characters  are  at  bottom  primary,  and  are  expressions  of  the 
same  general  habit  of  body  (or  to  use  the  medical  term, 
diathesis),  as  that  which  results  in  the  production  of  male 
elements  in  the  one  case,  or  female  elements  in  the  other."^ 

Three  well-known  facts  must  be  recalled  to  the  reader's 
mind  at  this  point ;  and  firstly,  that  in  a  great  number  of  cases 
the  secondary  sexual  characters  make  their  appearance  step  by 
step  with  sexual  maturity  itself  When  the  animal — be  it  a 
bird  or  insect — becomes  emphatically  masculine,  then  it  is  that 
these  minor  outcrops  arc  exhibited.  Thus  the  male  bird  of 
paradise,  eventually  so  resplendent,  is  usually  in  its  youth 
comparatively  dull  and  female- like  in  its  colouring  and 
plumage.  Very  often  too,  whether  in  the  wedding-robe  of 
male  fishes  or  in  the  scent-glands  of  mammals,  the  character 
rises  and  wanes  in  the  same  rhythm  as  that  of  the  reproductive 
periods.     It  is  impossible  not  to  regard  at  least  many  of  the 

*  That  Mr  Wallace  has  adopted  the  same  explanation  of  the  different 
sexual  characters  in  his  new  I)ook,  has  l)een  already  pointed  out  (see  p.  II, 
note). 


THE    SEXES,    AND    CRITICISM    OF    SEXUAL    SELECTION.         23 

secondary  sexual  characters  as  part  and  parcel  of  the  sexual 
diathesis,  —  as  expressions  for  the  most  part  of  exuberant 
maleness.  Secondly,  when  the  reproductive  organs  are 
removed  by  castration,  the  secondary  sexual  characters  tend  to 
remain  undeveloped.  Thus,  as  Darwin  notes,  stags  never 
renew  their  antlers  after  castration,  though  normally  of  course 
they  renew  them  each  breeding"  season.  The  reindeer,  where 
the  horns  occur  on  the  females  as  well,  is  an  interesting  excep- 
tion to  the  rule,  for  after  castration  the  male  still  renews  the 
growth.  This  however  merely  indicates  that  the  originally  sexual 
characters  have  become  organised  into  the  general  life  of  the 
body.  In  sheep,  antelopes,  oxen,  Sec,  castration  modifies  or 
reduces  the  horns ;  and  the  same  is  true  of  odoriferous  glands. 
The  parasitic  crustacean  Sacciilina  has  been  shown  by  Delage 
to  effect  a  partial  castration  of  the  crabs  to  which  it  fixes  itself, 
and  the  same  has  been  observed  by  (jiard  in  other  cases.  In 
two  such  cases  an  approximation  to  the  female  form  of  appendage 
has  been  observed.  Lastly,  in  aged  females,  which  have  ceased 
to  be  functional  in  reproduction,  the  minor  peculiarities  of  their 
sex  often  disappear,  and  they  become  liker  males,  both  in 
structure  and  habits, — witness  the  familiar  case  of  "crowing 
hens." 

From  the  presupposition,  then,  of  the  intimate  connection 
between  the  sexuality  and  the  secondary  characters  (which  is 
indeed  everywhere  allowed),  it  is  possible  to  advance  a  step 
further.  Thus  in  regard  to  colour,  that  the  male  is  usually 
brighter  than  the  female  is  an  acknowledged  fact.  But  pig- 
ments of  many  kinds  are  physiologically  regarded  as  of  the 
nature  of  waste  products.  Such  for  instance  is  the  guanin,  so 
abundant  on  the  skin  of  fishes  and  some  other  animals. 
Abundance  of  such  pigments,  and  richness  of  variety  in  related 
series,  point  to  pre-eminent  activity  of  chemical  processes  in  the 
animals  which  possess  them.  Technically  expressed,  abundant 
pigments  are  expressions  of  intense  metabolism.  But  pre- 
dominant activity  has  been  already  seen  to  be  characteristic  of 
the  male  sex ;  these  bright  colours,  then,  are  often  natural 
to  maleness.  In  a  literal  sense  animals  put  on  beauty  for 
ashes,  and  the  males  more  so  because  they  are  males,  and  not 
primarily  for  any  other  reason  whatever.  We  are  well  aware 
that,  in  spite  of  the  researches  of  Krukenberg,  Sorby,  MacMunn, 
and  others,  our  knowledge  of  the  physiology  of  many  of  the 
pigments  is  still  very  scanty.     Yet  in  many  cases,  alike  among 


24  THE    EVOLUTION    OF    SEX. 

plants  and  animals,  pigments  are  expressions  of  disruptive 
processes,  and  are  of  the  nature  of  waste  products  ;  and  this 
general  fact  is  at  present  sufficient  for  our  contention,  that  bright 
colouring  or  rich  pigmenting  is  commonly  a  natural  expression  of 
the  male  constitution.  For  the  red  pigment  so  abundant  in 
the  female  cochineal  insect,  which  appears  to  be  of  the  nature 
of  a  reserve  and  not  a  waste  product,  and  for  similar  occurrences, 
due  exception  must  be  made. 

In  the  same  way,  the  skin  eruptions  of  male  fishes  at  the 
spawning  season  seem  more  pathological  than  decorative,  and 
may  be  directly  connected  with  the  sexual  excitement.  One 
instance  of  the  way  in  which  the  reproductive  maturity  is  known 
to  effect  a  by  no  means  obviously  related  result  may  be  given. 
Every  field  naturalist  knows  that  the  male  stickleback  builds  a 
nest  among  the  weeds,  and  that  he  weaves  the  material  together 
by  mucous  threads  secreted  from  the  kidneys.  The  little  animal 
is  also  known  to  have  strong  passions  ;  it  is  polygamous  in 
relation  to  its  mates,  and  most  pugnacious  in  relation  to  its 
rivals.  Professor  Mobius  has  shown  that  the  male  reproductive 
organs  (or  testes)  l)ecome  very  large  at  the  breeding  season,  and 
that  they  press  in  an  abnormal  way  upon  the  kidneys.  This 
encroachment  produces  a  pathological  condition  in  the  kidneys, 
and  the  result  is  the  formation  of  a  mucous  secretion,  somewhat 
similar  to  what  occurs  in  renal  disease  in  higher  forms.  To 
free  itself  from  the  irritant  pressure  of  this  secretion,  the  male 
rubs  itself  against  external  objects,  most  conveniently  upon  its 
nest.  Thus  the  curious  weaving  instinct  does  not  demand  or 
find  rationale  in  the  cumulative  action  of  natural  selection  upon 
an  inexplicable  variation,  and  is  traced  back  to  a  pathological  and 
mechanical  origin  in  the  emphatic  maleness  of  the  organism. 
The  line  of  variation  being  thus  given,  it  is  of  course  conceiv- 
able that  natural  selection  may  have  accelerated  it. 

So  too,  tliough  again  the  physiological  details  are  scanty,  the 
superabundant  growth  of  hair  and  feathers  may  be  interpreted, 
in  some  measure  through  getting  rid  of  waste  products,  for  we 
shall  see  later  how  local  katabolism  favours  cell  multiplication. 
Combs,  wattles,  and  skin  excrescences  point  to  a  predominance 
of  circulation  in  the  skin  of  the  feverish  males,  whose  tempera- 
tures are  known  in  some  cases  to  be  decidedly  higher  than 
those  of  the  females.  Even  skeletal  weapons  like  antlers  may 
be  similarly  interpreted  ;  while  the  exaggerated  activity  of  the 
scent-glands  is  another  expedient  for  cxc^-cting  waste. 


THE    SEXES,    AND    CRITICISM    OF    SEXUAL    SELECTION.        25 

In  regard  to  horns,  feathers,  and  the  hke,  in  association 
with  vigorous  circulation,  two  sentences  from  Rolph  may  be 
quoted: — -"  The  exceedingly  abundant  circulation,  which  peri- 
odically occurs  in  the  at  first  soft  frontal  protuberances  of 
stags,  admits  and  conditions  the  colossal  development  of  horn 
and  delicate  ensheathing  velvet.  ...  In  the  same  way, 
the  rich  flow  of  blood  in  the  feather  papillae  conditions  the 
immense  growth  of  the  feathers,  .  .  .  and  the  same  is  true 
of  hairs,  spines,  and  teeth." 


jNIale  (c),  Worker  (/>),  and  Queen  (a)  Ant.  —  From  Cliaiubcrss  Encyc,  after  Lubbock. 

Some  of  the  even  subtler  differences  between  the  sexes  are 
of  interest  in  illustrating  the  general  antithesis.  Thus  in  the 
love-lights  of  the  Italian  glow  insect  {Luciola),  the  colour  is 
said  to  be  identical  in  the  two  sexes,  and  the  intensity  is  much 
the  same.  That  of  the  female,  however,  who  is  in  other  repects 
rather  male-like  in  her  amatory  emotions,  is  more  restricted. 
It  is  interesting  further  to  notice,  that  the  rhythm  of  the  light 
in  the  male  is  more  rapid  and  the  flashes  are  briefer,  while  that 
of  the  female  is  longer  and  the  flashes  more  distant  and  tremu- 
lous. This  illustration  may  thus  serve,  in  conclusion,  as  a 
literally  illumined  index  of  the  contrasted  physiology  of  the 
sexes. 

§  5.  Sexital  Selection:  its  Limit  as  an  Expla7iatio7t. — ^^^e 
are  now  in  a  better  position  to  criticise  Mr  Darwin's  theory. 
On  his  view,  males  are  stronger,  handsomer,  or  more  emo- 
tional, because  ancestral  forms  happened  to  become  so  in  a 
slight  degree.     In  other  words,  the  reward  of  breeding  success 


26  THE    EVOLUTION    OF    SEX. 

gradually  perpetuated  and  perfected  a  casual  advantage. 
According  to  the  present  view,  males  are  stronger,  handsomer, 
"or  more  emotional,  simply  because  they  are  males, — i.e.^  of 
more  active  physiological  habit  than  their  mates.  In  phrase- 
ology which  will  presently  become  more  intelligible  and 
concrete,  the  males  live  at  a  loss,  are  more  katalwiic,—d\'s>- 
ruptive  changes  tending  to  preponderate  in  the  sum  of  changes 
in  their  living  matter  or  protoplasm.  The  females,  on  the 
other  hand,  live  at  a  profit,  are  more  anabolic^ — constructive 
processes  predominating  in  their  life,  whence  indeed  the 
capacity  of  bearing  offspring. 

No  one  can  dispute  that  the  nutritive,  vegetative,  or  self- 
regarding  processes  within  the  plant  or  animal  are  opposed  to 
the  reproductive,  multiplying,  or  species-regarding  processes,  as 
income  to  expenditure,  or  as  building  up  to  breaking  down. 
But  within  the  ordinary  nutritive  or  vegetative  functions  of  the 
body,  there  is  necessarily  a  continuous  antithesis  between  two 
sets  of  processes,— constructive  and  destructive  metabolism. 
The  contrast  between  these  two  processes  is  seen  throughout 
nature,  whether  in  the  alternating  phases  of  cell  life,  or  of 
activity  and  repose,  or  in  the  great  antithesis  between  growth 
and  reproduction ;  and  it  is  this  same  contrast  which  we 
recognise  as  the  fundamental  difference  between  male  and 
female.  The  proof  of  this  will  run  through  the  work,  but  our 
fundamental  thesis  may  at  once  be  roughly  enunciated  in  a 
diagrammatic  expression  (which  in  its  present  form  we  owe  to 
our  friend  Mr  W.  E.  Fothergill)  : — 

SUM    OF    FUNC'l'IONS. 


Nuirition.  Rci)rocluction. 


\ 


Aiiabolism.  Katabolisin.  Female.  Male. 


Here    the    sum-total    of   the    functions    are    divided    into 
nutritive    and    reproductive,    the    former   into    anabolic    and 


THE    SEXES,    AND    CRITICISM    OF    SEXUAL    SELECTION.         27 

katabolic  processes,  the  latter  into  male  and  female  activities, — so 
far  with  all  physiologists,  without  exception  or  dispute.*  Our 
special  theory  lies,  however,  in  suggesting  the  parallelism  of  the 
two  sets  of  processes, — the  male  reproduction  is  associated  with 
preponderating  katabolism,  and  the  female  with  relative  anabol- 
ism.  In  terms  of  this  thesis,  therefore,  both  primary  and  second- 
ary sexual  characters  express  the  fundamental  physiological  bias 
characteristic  of  either  sex.  Sexual  selection  resembles  artificial 
selection,  but  the  female  takes  the  place  of  the  human  breeder;  it 
resembles  natural  selection,  but  the  selective  females  and  the  com- 
bative males  represent  a  role  filled  in  the  larger  case  by  the  foster- 
ing or  eliminating  action  of  the  environment.  As  a  special  case  of 
natural  selection,  Darwin's  minor  theory  is  open  to  the  objection 
of  being  teleological,  i.e.,  of  accounting  for  structures  in  terms  of 
a  final  advantage.  It  is  quite  open  to  the  logical  critic  to  urge, 
as  a  few  have  done,  that  the  structures  to  be  explained  have  to 
be  accounted  for  before,  as  well  as  after,  the  stage  when  they 
were  developed  enough  to  be  useful.  The  origin,  or  in  other 
words  the  fundamental  physiological  import,  of  the  structures, 
must  be  explained  before  we  have  a  complete  or  adequate  theory 
of  organic  evolution. 

Apart  from  this  logical  insufficiency,  the  theory  of  sexual 
selection  is  open  to  many  minor  objections,  with  some  of  which 
Darwin  himself  dealt,  as  is  mentioned  in  the  preceding  historical 
chapter.  One  detailed  objection  which  seems  serious  may  also 
be  urged.  The  evolution  of  coloured  markings  by  selective  pre- 
ference carries  with  it  the  postulate  of  a  certain  level  of  aesthetic 
taste  and  critical  power  in  the  female,  and  this  not  only  very 
high  and  very  scrupulous  as  to  details,  but  remaining  permanent 
as  a  standard  of  fashion  from  generation  to  generation, — large 
assumptions  all,  and  scarcely  verifiable  in  human  experience. 
Yet  we  cannot  suppose  that  Mr  Darwin  considered  the  human 
female  as  peculiarly  undeveloped.     It  is  true,  doubtless,  that 

*  The  reader  whose  physiological  studies  may  not  have  been  so  recent 
as  to  familiarise  him  with  that  conception  of  all  physiological  processes  as 
finding  their  ultimate  expression  in  the  metabolism  (anabolism  and 
katabolism)  of  protoplasm,  will  easily  place  himself  in  a  position  to  check 
our  argument  (often  indeed,  we  trust  to  carry  our  interpretation  of  sex  into 
still  further  detail)  by  starting  from  the  exposition  of  this  doctrine  in  Dr 
Michael  Foster's  article,  "  Physiology,"  in  the  Eiicycloptrdia  Britannica, 
or  with  Dr  Burdon  Sanderson's  Presidential  Address  to  Section  D,  British 
Association,  1889.  The  essential  conception  will,  however,  become  clearer 
as  we  proceed  (see  pp.  89,  124). 


28  THE    EVOLUTION    OF    SEX. 

both  insects  and  birds  have  so  far  and  increasingly  become 
educated  in  such  sensitiveness;  but  when  we  consider  the  com- 
plexity of  the  markings  of  the  male  bird  or  insect,  and  the  slow 
gradations  from  one  stage  of  perfection  to  another,  it  seems 
difficult  to  credit  birds  or  butterflies  with  a  degree  of  aesthetic 
development  exhibited  by  no  human  being  without  both  special 
aesthetic  acuteness  and  special  training.  Moreover,  the  butter- 
fly, which  is  supposed  to  possess  this  extraordinary  development 
of  psychological  subtlety,  will  fly  naively  to  a  piece  of  white 
paper  on  the  ground,  and  is  attracted  by  the  primary  aesthetic 
stimulus  of  an  old-fashioned  wall-paper,  not  to  speak  of  the 
gaudy  and  monotonous  brightness  of  some  of  our  garden  flowers. 
Thus  we  have  the  further  difficulty,  that  we  must  suppose  the 
female  butterfly  to  have  a  double  standard  of  taste,  one  for  the 
flowers  which  she  and  her  mate  both  visit,  the  other  for  the 
far  more  complex  colouring  and  markings  of  the  males.  And 
even  among  birds,  if  we  take  those  unmistakable  hints  of  real 
awakening  of  the  aesthetic  sense  which  are  exhibited  by  the 
iVustralian  bowerbird  or  by  the  common  jackdaw  in  its  fondness 
for  bright  objects,  how  very  rude  is  this  taste  compared  with  the 
critical  examination  of  infinitesimal  variations  of  plumage  on 
which  Darwin  relies.  Is  not,  therefore,  his  essential  supposition 
too  glaringly  anthropomorphic  ? 

Again,  the  most  beautiful  males  are  often  extremely  com- 
bative ;  and  on  the  conventional  view  this  is  a  mere  coin- 
cidence, yet  a  most  unfortunate  one  for  Mr  Darwin's  view. 
Battle  thus  constantly  decides  the  question  of  pairing,  and  in 
cases  where,  by  hypothesis,  the  female  should  have  most  choice, 
she  has  simply  to  yield  to  the  victor.  On  our  view,  however, 
combative  energy  and  sexual  beauty  rise  pari  passu  with  male 
katabolism. 

Or  again,  in  the  y^neas  group  of  the  genus  Fapilio,  Darwin 
notes  how  there  are  frequent  gradations  in  the  amount  of  dif- 
ference between  the  sexes.  Sometimes  the  sexes  are  alike  dull, 
where  we  should  have  to  suppose  the  aesthetic  perception  must 
somehow  have  been  lost  or  inhibited ;  sometimes  the  females 
are  dull  and  the  males  splendid, — for  Darwin,  an  example  of 
the  result  of  sexual  esthetic  perception,  this  of  an  exquisitely 
subtle  kind  however,  and  without  i)roportionate  cerebral  en- 
largement. In  a  third  set  of  cases,  both  sexes  arc  splendid, 
which  would  suggest  logically  that  the  male  in  turn  had  acquired 
a  taste  for  splendour.     But  such  cases,  which  usually  need  more 


THE    SEXES,    AND    CRITICISM    OF    SEXUAL    SELECTION.         29 

or  less  cumbrous  additional  hypothesis  of  inheritance  and  so  on 
to  explain  them,  are  intelligible  enough  if  we  regard  them  as 
illustrations  of  increasing  katabolism  throughout  a  series  of  species. 
The  third  set  may  be  supposed  to  be  more  male  or  katabolic 
than  the  first,  while  the  second  set  are  midway ;  although  it 
may  be  freely  granted  a  knowledge  of  the  habits,  size,  <S:c.,  of 
the  particular  species,  would  be  necessary  to  verify  the  legitimacy 
of  this  interpretation  in  this  particular  case.* 

It  is  necessary  once  more  to  turn  to  the  contrast  between 
the  positions  of  Darwin  and  Wallace.  According  to  Darwin, 
sexual  selection,  for  love's  sake,  has  accelerated  the  males  into 
gay  colouring ;  according  to  Wallace,  natural  selection,  for 
safety's  sake,  has  retarded  the  females  (birds  or  butterflies)  and 
kept  them  inconspicuously  plain.  It  is  no  longer  difficult  to 
establish  a  compromise.  The  true  view  seems  to  be,  that  both 
sexes  have  differentiated  towards  their  respective  goals,  but  the 
males  faster,  because  so  katal)olic;  the  limits  are  constantly  being 
fixed  by  natural  selection  in  Wallace's  cases,  and  as  constantly 
increased  by  sexual  selection  in  Darwin's.  There  is,  in  fact,  no 
reason  why  both  should  not  be  admitted  as  minor  factors ;  but 
the  greater  part  of  the  explanation  is  to  be  found  in  the  view 
above  stated,  viz.,  in  the  physiological  constitution  of  males  and 
females  themselves.  In  short,  the  present  position  allows  some 
truth  in  both  these  conclusions,  but  regards  gay  colouring  as 
the  expression  of  the  ])redominantly  katabolic  or  male  sex,  and 
quiet  plainness  as  equally  natural  to  the  predominantly  anabolic 
females.  On  this  view,  too,  we  are  able  to  restate  part  of  the 
position  emphasised  by  Brooks.  The  greater  variability  of  the 
males  is  indeed  natural,  if  they  be  the  more  katabolic  sex.  In 
preponderant  katabolism,  the  coml)inations  and  permutations 
of  molecules  which  constitute  variation,  are  necessarily  more 
probable  than  in  the  quiescent,  passive,  or  anabolic  females. 
No  special  theory  of  heredity  is  required, — the  males  transmit 
the  majority  of  variations,  because  they  have  most  to  transmit. 

At  a  later  stage  something  more  will  be  said  of  natural 
selection,   and  its  limits  as  an   explanation  of  facts.     But   it 

*  For  a  discussion  of  the  progressive  development  of  colouring  and 
markings,  whether  in  butterflies  or  mammals,  the  reader  may  be  referred 
to  the  works  of  Professor  Eimer,  and  especially  to  his  forthcoming  work 
on  Lejiidoptera.  Reference  should  also  be  made  to  Weismann's  "  Studies 
in  the  Theory  of  Descent,"  for  a  discussion  of  the  markings  of  caterpillars 
and  butterflies. 


30  THE    EVOLUTION    OF    SEX. 

is  here  desirable  to  emphasise,  that  just  as  we  admit  the 
importance  of  sexual  selection  as  a  minor  accelerant  in  the 
differentiation  of  the  sexes,  so  we  are  bound  to  recognise  that 
natural  selection  is  also  continually  in  operation  as  a  check  to 
a  divergence  of  the  sexes  which  would  otherwise  tend  to 
become  extreme.  If  this  retarding  influence  of  natural  selec- 
tion on  the  evolutionary  process  were  not  continually  present, 
we  should  find  cases  like  bonellia  and  the  rotifers  much 
commoner  than  they  are  among  animals.  But  it  is  an  error  to 
exaggerate  this  limiting  action  into  an  explanation  of  the 
process  itself.  It  should  also  be  noted,  that  both  the  retarding 
action  of  natural  selection,  and  the  accelerant  action  of  sexual 
selection,  become  of  increasing  importance  as  we  ascend  the 
series.  And  thus,  indeed,  we  are  impelled  towards  a  heresy 
which,  as  we  shall  see  later,  has  bearings  against  the  theory  of 
natural  selection,  which  overpass  the  limits  of  our  present 
theme. 

Postscript,— Y)xT.  W.  I'ulton,  Naturalist  to  the  Scottish  Fishery  Board, 
has  been  good  enough  to  furnish  us  with  some  of  his  results  on  the  size 
and  numerical  proportions  of  male  and  female  fishes,  (i.)  The  females 
are  usually  considerably  more  numerous  than  the  males,  and  never  less 
numerous  except  in  the  angler  and  the  cat-fish.  The  proportions  of  females 
to  males  among  flat-fishes  ranges  from  about  i  :  i  in  the  flounder,  to  about 
12  :  1  in  the  long  rough  dab.  Among  "  round  "  fishes  the  same  proportion 
varies  from  about  3  :  2  in  the  cod,  to  9  :  2  in  the  common  gurnard.  (2.)  The 
female  is  longer  and  larger  among  all  the  flat-fishes,  sometimes  by  as  much  as 
30  per  cent.  In  cod,  haddock,  angler,  and  cat-fish,  the  males  are  larger, 
while  in  the  whiting  the  females  are  slightly  larger,  and  in  the  common 
gurnard  decidedly  so.  The  subject  is  being  worked  up  by  the  above- 
named  naturalist,  and  cannot  fail  to  yield  very  valuable  results. 


THE    SEXES,    AND    CRITICISM    OF    SEXUAL    SELECTION.        3 1 


SUMMARY. 

I  3.  A  broader  basis  must  be  sought  from  which  to  understand  the 
differences  between  the  sexes.  A  general  survey  shows,  that  the  males  are 
more  active  in  habit,  the  females  more  passive  ;  that  the  males  tend  to  be 
smaller  and  to  have  a  higher  body-temperature,  while  the  females  tend  to 
be  larger  and  to  live  longer. 

4.  The  close  association  of  secondary  sexual  characters  with  the 
reproductive  function,  is  shown  in  the  period  or  in  the  periodicity  of  their 
development,  in  the  effects  of  castration,  in  the  peculiarities  of  aged 
females,  &c.  Richer  pigmentation,  and  other  male  characteristics,  are  to 
be  interpreted  as  expressions  of  the  katabolic  predominance  in  the  con- 
stitution of  males,  as  opposed  to  the  anabolic  preponderance  of  the 
females. 

5.  Sexual  selection,  as  an  explanation  of  secondary  sexual  characters, 
is  limited,  by  being  teleological  rather  than  retiological,  does  not  account 
for  origins  nor  incipient  stages,  postulates  subtle  resthetic  sensitiveness,  and 
is  beset  by  numerous  minor  difficulties.  Yet  the  opposed  positions  of 
Darwin  and  Wallace  both  emphasise  indubitable  facts;  while  the  criticisms 
of  Mivart,  the  theory  of  Brooks,  and  the  suggestions  of  Rolph,  Mantegazza, 
and  others,  lead  on  towards  a  deeper  analysis.  The  general  conclusion 
reached,  recognises  sexual  selection  (so  far  with  Darwin)  as  a  minor 
accelerant,  natural  selection  (so  far  with  \Yallace)  as  a  retarding  "  brake," 
on  the  differentiation  of  sexual  characters,  which  essentially  find  a  con- 
stitutional or  organismal  origin  in  the  katabolic  or  anabolic  diathesis 
which  preponderates  in  males  and  females  respectively. 


LITERATURE. 

Brooks,  Darwin,  Mivart,  Wallace. — As  before. 

EiMER,  G.  H.  T. — Die  Enstehung  der  Arten  auf  Grund  von  Vererben 
erworbener  Eigenschaften,  nach  den  Gesetzen  organischen  Wachsens. 
Jena,  1SS8. 

Geddes,  p. — Articles  Reproduction,  Sex,  Yariation  and  Selection. 
Encycl.  Brit.  Also  on  the  Theory  of  Growth,  Reproduction,  Sex, 
and  Heredity.      Proc.  Roy.  Soc.     Edin.  1S85-6. 

RoLPH,  W.  H. — Biologische  Probleme.     Leipzig,  1S84. 

Weismann,  a. — Studies  in  the  Theory  of  Descent  (Meldola's  Transla- 
tion).    London,  1880-82. 

Wallace,  A.  R.— Darwinism.     London,  1889. 


CHAPTER     III. 

The  Determination  of  Sex  (^Hypotheses  and  Observations). 

So  far  the  differences  between  the  sexes  as  observed  in  adult 
forms.  Attention  must  now  be  turned  to  the  origin  of  sex  itself 
in  the  individual  organism.  The  historic  beginning  of  sex  will 
be  discussed  at  a  later  stage ;  the  present  problem  concerns 
the  factors  which  determine  whether  any  given  organism  will 
develop  into  a  male  or  into  a  female.  The  question,  in  other 
words,  is  that  usually  known  as  the  determination  of  sex. 

§  I.  The  Period  at  wiiich  the  Sex  is  Determined. — Every 
organism,  whether  male  or  female,  develops  from  a  fertilised 
egg-cell,  apart  of  course  from  the  occurrence  of  asexual  and 
parthenogenetic  reproduction.  This  material,  which  in  one 
case  develops  into  a  male,  in  another  into  a  female,  is,  so  far 
as  our  experience  can  go,  always  the  same  ;  and  when  the  sex  of 
the  organism  is  absolutely  decided,  is  a  question  to  which  no 
general  answer  can  be  given.  In  the  higher  animals  (birds 
and  mammals)  it  is  possible  at  quite  an  early  date  in  embryonic 
life  to  tell  whether  the  young  organism  will  turn  into  a  niale  or 
a  female,  though  in  the  very  earliest  stages  it  is  impossible  to 
determine  whether  the  rudiment  of  the  reproductive  organs  is 
going  to  become  a  testis  or  an  ovary.  But  in  lower  vertebrates, 
such  as  frogs,  the  period  of  embryonic  indifference  is  greatly 
prolonged ;  and  it  seems  certain  that  a  hatched  tadpole,  even 
after  a  tendency  towards,  say  maleness,  has  actually  arisen, 
may  in  certain  conditions  have  this  altered  in  the  opposite 
direction.  Among  invertebrates,  the  sexual  organs  are  often 
late  in  acquiring  definite  predominance  in  favour  of  either  sex, — 
that  is,  the  period  of  undecided  indifference  is,  as  one  would 
expect,  usually  much  longer. 

The  factors  which  are  influential  in  determining  sex  are 
numerous,  and  come  into  play  at  different  periods,  so  that  it 
is  quite  possible  for  a  germ-cell  to  have  its  future  fate  more 
than  once  changed.  The  constitution  of  the  mother,  the 
nutrition  of  the  ova,  the  constitution  of  the  father,  the  state  of 


tup:  determination  of  sex.  ^^ 

the  male  element  when  fertilisation  occurs,  the  embryonic 
nutrition,  and  even  the  larval  environment  in  some  cases, 
these  and  yet  other  factors  have  all  to  be  considered. 

Some  observations  by  Laulanie  as  to  the  embryonic  organs  are  of 
interest  in  this  connection.  He  distinguishes  both  in  birds  and  mammals 
three  stages  in  the  individual  development  of  the  reproductive  organs. 
These  he  calls  (i)  Germiparity,  (2)  Hermaphroditism,  (3)  Differentiated 
Unisexuality  ;  and  regards  them  as  parallel  to  the  stages  of  historic  evolu- 
tion. Even  for  the  first  stage,  however,  when  the  elements  are  still  very 
primitive,  he  would  not  allow  the  accuracy  of  the  terms  neutrality  or 
indifference.  The  elements  in  both  sexes  are  almost  similar,  but  yet  their 
future  fate  has  been  decided. 

Sutton  has  also  emphasised  his  conviction,  that  in  the  individual 
development  a  state  of  embryonic  hermaphroditism  obtains,  and  main- 
tains that  one  set  of  elements  predominates  over  the  other  in  the  establish- 
ment of  the  normal  unisexual  state.  Ploss  and  others  take  up  a  similar  position 
in  regard  to  an  early  hermaphrodite  state.  It  can  only  be  concluded,  that 
the  higher  the  organism  is  in  the  series  the  earlier  is  its  sexual  fate  sealed  ; 
and  that  it  is  only  in  lower  vertebrates,  and  among  backboneless  animals, 
that  we  can  speak  of  prolonged  neutrality  of  sex,  or  embryonic  hermaphro- 
ditism. 

§  2.  Ansivers  to  the  Question:  What  Deter/nines  Sex? — To 
the  question  what  settles  whether  an  organism  shall  develop 
into  a  male  or  into  a  female,  many  and  varied  answers  have 
been  given.  At  the  beginning  of  the  last  century,  the  theories 
of  sex  were  estimated  at  so  many  as  five  hundred,  and  they  have 
gone  on  increasing.  It  is  evident  that  even  an  enumeration  of 
these  is  not  possible,  nor  is  it  indeed  desirable.  As  in  so 
many  other  cases,  our  ideas  respecting  the  determination  of  sex 
have  been  looked  at  in  three  different  ways.  For  the  theologian, 
it  was  enough  to  say  that  "God  made  male  and  female."  In 
the  period  of  academic  metaphysics,  still  so  far  from  ended,  it 
was  natural  to  refer  to  "inherent  properties  of  maleness  and 
femaleness;"  and  it  is  still  a  popular  "explanation"  to  invoke 
undefined  "  natural  tendencies  "  to  account  for  the  production 
of  males  or  females.  This  mode  of  treatment,  it  need  not  be 
said,  is  being  abandoned  by  biologists.  It  is  recognised  that 
the  problem  is  one  for  scientific  analysis ;  thus  the  constitu- 
tion, age,  nutrition,  and  environment  of  the  parents  must  be 
especially  considered.  These  investigations,  which  are  mainly 
restricted  to  observation  and  statistics,  will  be  first  noticed ;  the 
more  experimental  researches,  and  the  general  conclusions,  will 
be  discussed  in  the  next  chapter.  That  the  final  physiological  ex- 
planation is,  and  must  be,  in  terms  of  protoplasmic  metabolism, 
we  must  again,  however,  remind  the  reader  (see  p.  27,  note). 

c 


34 


THE    EVOLUTION    OF    SEX. 


§  3.  The  theory  that  there  are  two  kinds  of  ova,  respectively 
destined  to  develop  into  males  or  females,  is  more  than  a 
mere  begging  of  the  question.  The  constitution  of  the  ovum 
is  undoubtedly  a  fact  of  primal  importance,  but  we  must  also 
recognise  that  what  is  virtually  decided  at  this  early  stage  may 
be  counteracted  by  later  influences  of  an  opposite  character. 
The  hypothesis  of  two  kinds  of  ova  was  advanced,  for  example, 
by  B.  S.  Schultze,  but  as  the  grounds  for  his  views  are  not 
admitted  as  correct,  only  its  existence  need  be  noticed  till  more 
observations  are  forthcoming. 

§  4.  Numerous  authors  have  attached  great  importance  to 
the  process  of  fertilisation  as  a  determinant  of  the  sex. 

One  of  the  most  crude  positoins  has  been  that  of  Canestrini,  who 
ascribed  the  determination  of  sex  to  the  number  of  sperms  entering  the 
ovum  : — The  more  sperms,  the  greater  the  tendency  to  male  offspring.  It 
has,  however,  been  shown  by  Fol,  Pfliiger,  Hertwig,  and  others,  that 
"polyspermy,"  or  the  entrance  of  more  than  one  sperm,  is  extremely  rare, 
is  in  fact  generally  impossible,  and  when  it  does  in  rare  conditions  occur, 
indicates  a  pathological  condition  of  the  egg-cell,  and  tends  to  produce 
abnormalities.  Pfliiger  diluted  the  seminal  fluid  of  male  frogs,  and 
found  that  no  change  resulted  in  the  normal  numerical  proportion  of 
the  sexes.  The  case  of  drones,  furthermore,  where  male  are  known  to 
arise  from  unfertilised  ova,  is  a  familiar  example,  exactly  counter  to 
Canestrini's  proposition,  which  may  in  fact  be  dismissed  as  wholly 
untenable. 

§  5.  Time  of  Fertilisation. — With  greater  weight  various  authorities 
have  insisted  upon  the  time  of  fertilisation.  Thus,  according  to  Thury 
(1863),  followed  by  Dusing  (1883),  an  ovum  fertilised  soon  after  liberation 
tends  to  produce  a  female,  while  an  older  ovum  will  rather  develop  into  a 
male.  As  a  practical  breeder  Thury  claimed  to  determine  the  sex  of  cattle 
upon  this  principle  ;  Cornaz  and  Knight  have  both  practically  confirmed 
this  ;  while  Girou  has  pointed  out,  that  female  flowers  fertilised  as  soon  as 
they  were  able  to  receive  pollen  tended  to  produce  female  offspring. 
Hertwig  has  also  shown  that,  the  internal  phenomena  of  fertilisation  vary 
somewhat  with  the  age  of  the  ovum  at  the  time.  Hensen  is  inclined  to 
accept  the  general  accuracy  of  Thury's  conclusion,  but  extends  it  to  the 
male  element  as  well.  "A  very  favourable  condition  in  both  ovum  and 
sperm  will  probably  lead  to  the  formation  of  a  female."  "  According  to 
its  condition,  a  sperm  may  either  insufficiently  corroborate  the  favourable 
state  of  the  ovum,  or  constitutionally  strengthen  an  ovum  less  satisfactorily 
conditioned." 

§  6.  Age  of  Parents. — Ilofackcr  (1823)  and  Sadler  (1830)  indei")endently 
published  a  body  of  statistics,  each  including  about  2,000  births,  in  favour 
of  the  generalisation  that  when  the  male  parent  is  the  older  the  offspring 
are  preponderatingly  male  ;  while  if  the  parents  be  of  the  same  age,  or 
a  fortiori  if  the  male  parent  be  the  younger,  female  offspring  appear  in 
increasing  majority.  This  conclusion,  generally  known  as  Ilofacker's  and 
Sadler's  law,  has  received  both  confirmation  and  perplexing  contradiction. 
It  has  been  confirmed  by  Gohlert,  Boulenger,  Legoyt,  and  others,  and  by 


THE    DETERMINATION    OF    SEX. 


35 


some  breeders  of  stock  and  birds,  but  is  denied  by  other  practical 
authorities,  and  directly  contradicted  by  the  recent  statistics  of  Stieda, 
from  Alsace-Lorraine,  and  of  Berner,  from  Scandinavia. 


Summary  of  Statistics  bea 

ring  on  Relative  N 

umber  of  Males  and  Females. 

Observer. 

No.  of 
Births. 

Locality. 

Father 

older. 

Proportion 

of  Males  to 

100  Females. 

Father  of 
equal  age. 
Proportion 
of  Males  to 
100  Females. 

Father 

younger. 

Proportion 

of  Males  to 

100  Females. 

90-6 

Average 
Propor- 
tion of 

Males  to 
100 

Females. 

Remarks. 

Hofacker 

1,996 

Tiibingen 

117.8 

92.0 

107.S 

Sadler 

2,068 

England 

121.4 

94.8 

86.5 

114.7 

GOhlert 

4,584 

108.2 

93-3 

82.6 

105.3 

Legoyt 

52,311 

Paris 

104.49 

102.14 

97-5 

102.97 

Boulenger 

6,006 

Calais 

109.98 

107.92 

101.63 

107.9 

Noirot 

4,000 

Dijon 

99-7 

116.0 

103.5 

Breslau 

8,084 

Zurich 

103.9 

103.1 

1 1 7. 6 

186.6 

Stieda 

100,590 

Alsace- 
Lorraine 

105.03 

108.39 

106.27 

Contradictory. 

Berner 

267,946 

Sweden 

104.61 

106.23 

107.45 

106.0 

Contradictory 
(see  text). 

The  above  table  (in  its  upper  part  taken  mainly  from  Hensen,  after 
GLsterlen)  shows  vividly  how  much  the  results  of  Stieda  and  Berner 
conflict  with  the  law  of  Hofacker  and  Sadler.  In  regard  to  Berner's 
statistics,  it  ought  to  be  further  noted  that  the  figures  quoted  refer  to  cases 
where  the  father  or  mother  is  only  from  I  to  10  years  the  older.  If  the 
father  be  more  than  ten  years  older,  the  male  majority  is  103.54  ;  if  the 
mother  be  more  than  ten  years  older,  the  proportion  is  104.10  again, 
against  Hofacker's  and  Sadler's  conclusion.  Compared  with  the  above 
human  statistics,  Schlechter's  results  in  regard  to  horses  also  militate 
against  the  alleged  law. 

In  regard  to  plants,  various  naturalists  have  drawn  attention  to  the 
influence   of  age  upon   sex.     The   following  observations   are   quoted    by 


36  THE    EVOLUTION    OF    SEX. 

Heycx  :  —In  Leontarus  domestica,  according  to  Rumpf,  the  female  plant 
may  bear  male  blossoms  l:»efore  its  proper  female  flowers.  In  Morns  nigra^ 
and  in  other  cases,  according  to  Miller,  male  flowers  may  be  borne  first, 
and  afterwards  fruit.  Treviranus  observed  that  the  first  flowers  of  beech, 
chestnut,  and  other  trees  are  male.  Clausen  gives  similar  examples  ;  and 
Hoftmann  notes  that  in  the  horse-chestnut,  and  several  other  cases,  male 
flowers  appear  first,  and  afterwards  hermaphrodites  or  females. 

Most  of  the  results  in  regard  to  the  influence  of  age  are, 
however,  extremely  unsatisfactory  and  conflicting.  This  is 
evident  from  the  above  statistics.  The  law  of  Hofacker  and 
Sadler  cannot  be  regarded  as  in  any  sense  established.  In 
fact,  as  Hensen  remarks,  unless  statistics  are  enormously  large 
they  prove  very  little.  7^he  number  of  other  factors  besides 
parental  age  which  may  operate  in  any  case  is  evidently  great, 
— health,  nutrition,  frequency  of  sexual  intercourse,  abstinence 
after  the  birth  of  a  male,  and  the  like,  all  reduce  the  feasibility 
of  the  statistical  method.  At  present,  at  any  rate,  we  are  not 
justified  in  ascribing  much  importance  to  the  relative  age  of  the 
parent  except  as  a  secondary  factor,  influential  doubtless  in 
relation  to  nutrition. 

§  7.  Comparative  Vigour. — The  best  known,  and  probably 
still  most  influential,  theory  is  that  of  "  comparative  vigour." 
As  elaborated  by  Girou  and  others,  this  hypothesis  connects 
the  sex  of  the  offspring  with  that  of  the  more  vigorous  parent. 
It  cannot  be  said,  however,  that  facts  bear  out  the  case.  Thus 
consumptive  mothers  produce  a  great  excess  of  daughters, 
w^hile  Girou's  theory  would  lead  us  to  expect  the  opposite. 
We  require  in  fact  to  have  "  vigour "  analysed  out  into  its 
component  factors,  and  in  so  doing  we  shall  afterwards  find 
not  only  facts  but  reasons  in  favour  of  the  conclusion,  in  part 
included  in  the  above  theory,  that  highly  nourished  females 
tend  to  produce  female  offspring.  That  form  of  the  hypothesis 
which  refers  the  determination  of  sex  to  "  genital  superiority," 
or  to  "  relative  ardency,"  can  hardly  be  seriously  considered. 
In  this  connection  it  has  been  maintained  that  in  "  marriages  of 
love,"  after  a  short  betrothal,  female  offspring  predominate ; 
and  a  number  of  other  interesting  facts  of  a  like  nature  are 
suggested.  Some  scepticism  as  to  the  practicabihty  of  such 
inductions  is,  however,  inevitable. 

v^  8.  Starkiveather' s  Law  of  Sex. — Closely  allied  to  the 
theory  of  comparative  vigour  is  that  elaborately  worked  out  by 
Starkweather,  which  is  suggestive  enough  to  deserve  separate 
summary.     He  starts  from  a  discussion  of  the  alleged  superiority 


THE    DETERMINATION    OF    SEX.  37 

of  either  sex.  Few  maintain  that  the  sexes  are  essentially 
equal,  still  fewer  that  the  females  excel ;  the  general  bias  of 
authority  has  been  in  favour  of  the  males.  From  the  earliest 
ages  philosophers  have  contended  that  woman  is  but  an 
undeveloped  man ;  Darwin's  theory  of  sexual  selection  pre- 
supposes a  superiority  and  an  entail  in  the  male  line ;  for 
Spencer,  the  development  of  woman  is  early  arrested  by  pro- 
creative  functions.  In  short,  Darwin's  man  is  as  it  were  an 
evolved  woman,  and  Spencer's  woman  an  arrested  man. 

This  notion  of  the  superiority  of  males  has  formed  the  basis 
of  many  theories  of  sex.  As  a  good  illustration  of  this  opinion, 
a  few  sentences  may  be  quoted  from  Richarz  : — "The  sex  is 
not  a  quality  transmitted  from  the  parents,  but  has  its  basis  in 
the  degree  of  organisation  attained  by  the  offspring.  The  male 
sex  represents  to  a  certain  extent  a  higher  grade  of  organisation 
or  development  in  the  embryo.  This  is  attained  when  the 
reproductive  efficiency  of  the  mother  is  specially  well  developed, 
and  the  resulting  male  offspring  more  or  less  resembles  the 
mother.  But  if  the  maternal  re[)roductive  power  be  weak,  the 
ovum  does  not  attain  to  maleness,  and  the  resulting  female 
offspring  more  or  less  resembles  the  father."  Thus  Hough 
thinks  males  are  born  when  the  maternal  system  is  at  its  best ; 
more  females  at  periods  of  growth,  reparation,  or  disease. 
Tiedman  and  others  regard  female  offspring  as  arrested  in  the 
original  state ;  while  Velpau  conversely  regards  females  as 
degenerate  from  primitive  maleness. 

Reacting  from  such  speculations  as  to  superiority  of  either 
sex.  Starkweather  firmly  maintains  that  "  neither  sex  is 
physically  the  superior,  but  both  are  essentially  equal  in  a 
physiological  sense."  This  is  true  in  the  average,  but  yet  in 
each  pair  a  greater  or  less  degree  of  superiority  on  one  side  or 
other  must  usually  be  conceded.  Granting  this,  Starkweather 
states,  as  his  chief  conclusion,  "  that  sex  is  determined  by  the 
superior  parent,  also  that  the  superior  parent  produces  the 
opposite  sex."  Referring  the  reader  to  the  Ency.  Brit.  i\rticle 
"  Sex,"  for  some  critical  notes,  it  is  enough  here  to  notice,  that 
just  like  "comparative  vigour,"  so  "superiority"  has  little  more 
than  verbal  simplicity  to  recommend  it,  since  it  lumps  a  great 
variety  of  factors  under  a  common  name.  Yet,  in  justice  to  its 
author,  we  may  admit  that  it  is  the  algebraic  sum  of  these 
which  he  aims  at  expressing. 

§  9.  Darwin's  Positiofi. — Neither  in  regard  to  the  origin  of 


38  THE    EVOLUTION    OF    SEX. 

sex,  nor  its  determination  in  individual  cases,  did  Darwin  see 
further  than  his  contemporaries.  He  refers  to  the  current 
theories  of  the  influence  of  age,  period  of  impregnation,  and 
the  hke ;  and  further  contributes  a  great  body  of  statistics  on 
the  numerical  proportions  of  the  sexes,  and  the  supposed 
influence  of  polygamy.  "There  is  reason,"  he  says,  "to 
suspect  that  in  some  cases  man  has  by  selection  indirectly 
influenced  his  own  sex-producing  powers."  He  falls  back 
upon  the  unanalysed  "belief  that  the  tendency  to  produce 
either  sex  would  be  inherited  like  almost  every  other  peculiarity, 
for  instance,  that  of  producing  twins."  "  In  no  case,  as  far  as 
we  can  see,  would  an  inherited  tendency  to  produce  both  sexes 
in  equal  numbers,  or  to  produce  one  sex  in  excess,  be  a  direct 
advantage  or  disadvantage  to  certain  individuals  more  than  to 
others  ;  .  .  .  and  therefore  a  tendency  of  this  kind  could 
not  be  gained  through  natural  selection."  "  I  formerly  thought 
that  when  a  tendency  to  produce  the  two  sexes  in  equal 
numbers  was  advantageous  to  the  species,  it  would  follow  from 
natural  selection,  but  I  now  see  that  the  whole  problem  is  so 
intricate  that  it  is  safer  to  leave  its  solution  for  the  future." 
Any  other  hints  that  Darwin  threw  out,  have  been  so  well 
elaborated  by  Diising's  work  on  the  advantageous  self-regula- 
tion of  the  sex-proportions,  that  reference  to  the  latter  is  more 
profitable. 

§  10.  D  a  sing  on  the  P7'Oportio7is  of  the  Sexes,  and  the 
Regulation  of  these.  —  In  an  important  work,  Diising  has 
recently  treated  the  whole  subject  with  some  synthetic  result. 
He  recognises  that  the  fates  or  factors  determining  the  sex  are 
manifold,  and  operate  at  different  periods.  Much  is  determined 
by  the  condition  of  the  reproductive  elements,  i.e.,  by  the  con- 
stitution and  habits  of  the  parents ;  much  depends  also  on  the 
period  of  fertilisation  ;  while  again  the  nutrition  of  the  embryo 
may  be  of  moment.  Diising  has  collected  a  great  body  of 
facts,  from  both  plants  and  animals,  in  favour  of  his  conclusions; 
but  the  copious  summary  of  his  work,  given  in  the  article  "Sex" 
already  referred  to,  need  not  here  be  repeated,  while  some  of 
his  experimental  results  will  be  included  in  the  next  chapter. 

Diising's  memoir  is  very  important,  however,  for  this  special 
reason,  that  he  analyses  what  may  be  termed  the  mechanism 
by  which  the  proportion  of  the  sexes  is  regulated.  Instead  of 
vaguely  referring  the  whole  matter  to  natural  selection,  he 
shows  in  detail  how  the  numbers  are  in  a  sense  self-regulating, 


THE    DETERMINATION    OF    SEX.  39 

how  there  is  always  produced  a  majority  of  the  sex  that  is 
wanted.  That  is  to  say,  if  one  sex  be  in  the  decided  minority, 
or  under  conditions  which  come  to  the  same  thing,  then  a 
majority  of  that  sex  will  be  produced.  If  there  be,  for  instance, 
a  great  majority  of  males,  there  is  the  greater  likelihood  of  the 
ova  being  fertilised  early,  but  that  means  a  probable  pre- 
ponderance of  female  offspring,  and  thus  the  balance  is 
restored.  It  would  be  rash  to  say  that  in  every  case  he  makes 
out  his  contention,  but  his  general  argument,  that  disturbances 
in  the  proportion  of  the  sexes  bring  about  their  own  compensa- 
tion, is  carefully  and  convincingly  worked  out. 

§  II.  Sex  of  TwiJis. — It  sometimes  happens  among  many  different 
classes  of  animals  that  from  one  ovum  two  organisms  develop.  We  have 
then  a  case  of  "  true"  twins,  as  opposed  to  cases  where  multiple  offspring 
do  not  arise  from  one  ovum.  Such  "  true "  twins  seem  to  occur  not 
uncommonly  in  the  human  species,  and  are  either  most  markedly  similar 
to  one  another  or  strongly  dissimilar.  The  import  of  this  is  one  of  the 
minor  problems  of  heredity,  and  cannot  be  here  discussed,  but  we  have  to 
note  the  general  fact,  which  holds  without  exception  in  the  human  species, 
that  "  true  "  twins  are  of  the  same  sex. 

From  a  very  early  date  an  exception  to  this  rule  has  been  known  in 
regard  to  cattle,  and  applies  to  some  other  organisms  as  well.  From  the 
careful  researches  of  Spiegelberg  and  others,  it  appears  that  in  cattle  {a) 
the  twins  may  be  both  female  and  then  both  normal,  or  {b)  that  the  sexes 
may  be  different  and  normal,  or  {c)  that  both  may  be  males,  in  which  case 
one  always  exhibits  the  peculiar  abnormality  known  as  a  "free-martin." 
The  internal  organs  are  male,  but  the  external  accessory  organs  are  female, 
and  there  are  also  rudimentary  female  ducts.  No  theory  has  yet  explained 
the  facts  of  this  case. 

It  is  now  necessary,  with  Diising  for  transition,  to  pass  from 
the  historical  mode  of  treatment  to  something  more  con- 
structive. Leaving  mere  hypotheses  behind,  as  well  as  theories 
based  on  insufficient  statistics,  an  induction  from  experimental 
evidence  will  be  built  up  in  the  following  chapter. 


40  THE    EVOLUTION    OF    SEX. 

SUMMARY. 

1.  The  epoch  at  which  the  sex  is  finally  determined  is  variable  in  different 
animals,  and  diverse  factors  operate  at  successive  epochs. 

2.  Theological  and  metaphysical  theories  of  sex  have  preceded  the 
scientific  ;  observation  and  statistics  have  been  resorted  to  before  experi- 
ment ;  and  over  500  theories  in  all  have  been  set  forth. 

3-6.  That  there  are  two  kinds  of  ova  is  still  for  the  most  part  an  assump- 
tion ;  that  the  entrance  of  more  than  one  spermatozoon  normally  occurs, 
and  is  a  determining  factor,  is  erroneous.  Thury's  emphasis  on  the  age  of 
the  ovum  when  fertilised  is  probably  justified  ;  while  Hensen  extends  this 
notion  to  the  male  element  as  well.  'I'he  age  of  the  parents  is  probably  only 
of  secondary  import,  and  the  law  of  Hofacker  and  Sadler  is  not  confirmed. 

7,  8.  Theories  of  "comparative  vigour  "  and  the  like  must  be  dismissed  ; 
while  Starkweather's  theory  of  the  relative  superiority  of  either  sex,  and  of 
the  influence  of  this  on  the  sex  of  the  offspring,  requires  further  analysis. 

9,  10.  Darwin's  position  contains  nothing  novel,  and  has  been  superseded 
by  Diising's  synthetic  treatment  and  explanation  of  the  self-regulating 
numerical  proportion  of  the  sexes. 

10.  From  this  point,  after  a  note  on  the  similar  sex  of  "true"  twins, 
we  pass  to  the  experimental  data  and  constructive  treatment. 

LITERATURE. 

Berner. — Hj.  Om  Kjonsdannelsens  Aarsager,  En  biologisk  Studie  (with 

numerous  references).     Christiania,  1883. 
Darwin,  C. — The  Descent  of  Man,  Chap.  VIII.     London,  1871. 

The  Variation  of  Animals  and  Plants  under  Domestication.   Lond. 

D USING,    C. — Die     Regulierung    des    Geschlechtsverhaltnisses    bei    der 

Vermehrung  der  Menschen,  Thiere,  und  Pflanzen.      Jena,  1884  ;  or, 

Jen.  Zeitsch.  f.  Naturw.,  XVII.,  1883. 
Geddes,  p. — As  before. 
Hensen,    V. — Physiologic    der    Zeugung.     Hermann's    Handbuch    der 

Physiologic,  Bd.  VI.,  pp.  304,  with  references  to  Ploss,  Schultze,  &c. 

Leipzig,  1 88 1. 
PIis,  W. — Theorien  der  geschlechtlichen  Zeugung.  Arch.  f.  Anthropologic. 

Bde.  IV. -VI. 
Hofacker. — Ueber  die  Eigenschaften,  welche  sich  bei  Menschen  und 

Thieren  auf  die  Nachkommen  vererben.     Tubingen,  1828. 
Laulani^,  F. — Comptes  Rendus,  CI.,  pp.  593-5.     1885. 
RoLPH,  W.  H. — As  before. 

Roth,  E. — Die  Thatsachen  der  Vererbung  (historical).     Berlin,  1885. 
Pfluger,  E. — Ueber  die  das  Geschlecht  bestimmenden  Ursachen  und  die 

Geschlechts-verhaltnisse  der  Frosche.    Arch.  f.  d.  ges.  Physiol.  XXIX. 
Sadler. — The  Law  of  Population.     London,  1830. 
ScHLECHTER. — Ucbcr  die  Ursachen  welche   das  Geschlecht  bestimmen. 

Rev.  f.  Tierheilkunde.    Wien,  1884.    Biol.  Centralblt.,  IV.,  pp.  627-9. 
Starkweather. — The  Law  of  Sex.     London,  1883. 
Stieda. — Das  Sexual  Verhaltniss  bei  Geborenen.     Strasburg,  1875. 
Sutton,  J.  B. — General  Pathology.     London,  1886. 

Thury. — Ueber  das  Gesetzder  Erzeugungder  Geschlcchter.    Leipzig,  1863. 
Wappceus. — Allgemeine  Bevolkerungs-Statistik.     Leipzig,  1861. 


CHAPTER    IV. 

The  Dp:termination  of  Sex. 
{Experiment  and  Rationale?) 

§  I.  Influence  of  Nutrition. — Throughout  nature  the  influence 
of  food  is  undoubtedly  one  of  the  most  important  environ- 
mental factors.  To  Claude  Bernard,  indeed,  the  whole  problem 
of  evolution  was  very  much  a  question  of  variations  of  nutrition. 
"  L'evolution,  c'est  I'ensemble  constant  de  ces  alternatives  de 
la  nutrition ;  c'est  la  nutrition  consideree  dans  sa  realite,  em- 
brassee  d'un  coup  d'oeil  a  travers  le  temps."  It  is  fitting  that 
we  should  begin  our  survey  of  the  factors  known  to  influence 
sex  with  the  fundamental  function  of  nutrition. 

{a)  The  Case  of  Tadpoles. — Not  a  few  investigators  who 
have  passed  from  statistics  and  hypothesis  to  experiment  and 
induction,  have  found  their  material  in  tadpoles,  where  the  sex 
seems  to  remain  for  a  comparatively  long  period  indeterminate. 
If  we  take  the  verdict  of  Yung,  who  has  had  most  experience 
with  these  forms,  tadpoles  pass  through  a  hermaphrodite  stage, 
in  common,  according  to  other  authorities,  with  most  animals. 
During  this  phase  external  influences,  and  especially  food,  decide 
their  fate  as  regards  sex,  though  the  hermaphroditism,  as  we 
shall  afterwards  see,  sometimes  persists  in  adult  life.  It  is  fair, 
however,  to  notice  that  Pfliiger  gives  a  somewhat  different 
account  of  the  actual  facts,  distinguishing  among  tadpoles  three 
varieties— (<?)  distinct  males,  (/>)  distinct  females,  and  {c)  herma- 
phrodites. In  the  last,  testes,  or  male  organs,  develop  round 
primitive  ovaries,  and  if  the  tadpoles  are  to  become  males  the 
enclosed  female  organs  are  absorbed. 

Adopting  the  view  stated  by  Yung,  we  shall  simply  state  the 
striking  results  of  one  series  of  observations.  When  the  tadpoles 
were  left  to  themselves,  the  percentage  of  females  was  rather  in 
the  majority.  In  three  lots,  the  proportions  of  females  to 
males  were  as  follows  : — 54  :  46  ;  61  :  39  ;  and  56  :  44.  The 
average  number  of  females  was  thus  about  57  in  the  hundred. 


42 


THE    EVOLUTION    OF    SEX. 


In  the  first  brood,  by  feeding  one  set  with  beef,  Yung  raised  the 
percentage  of  females  from  54  to  78;  in  the  second,  with  fish, 
the  percentage  rose  from  61  to  81  ;  while  in  the  third  set,  when 
the  especially  nutritious  flesh  of  frogs  was  supplied,  the  per- 
centage rose  from  56  to  92.  That  is  to  say,  in  the  last  case  the 
result  of  high  feeding  was  that  there  were  92  females  to  8  males. 
From  the  experience  and  carefulness  of  the  observer,  these 
striking  results  are  entitled  to  great  weight. 

{/>)  Case  of  Bees. — The  three  kinds  of  inmates  in  a  beehive 
are  known  to  every  one  as  queens,  workers,  and  drones ;  or,  as 
fertile  females,  imperfect  females,  and  males.  What  are  the 
factors  determining  the  differences  between  these  three  forms  ? 
In  the  first  place,  it  is  believed  that  the  eggs  which  give  rise  to 
drones  are  not  fertilised,  while  those  that  develop  into  queens 
and  workers  have  the  normal  history.  But  what  fate  rules  the 
destiny  of  the  two  latter,  determining  whether  a  given   ovum 


The  Queen  (a),  Worker  (c),  and  Drone  (n) 
of  the  Common  Hive-Bee, 

will  turn  out  the  possible  mother  of  a  new  generation,  or  remain 
at  the  low^er  level  of  a  non-fertile  working  female?  It  seems 
certain  that  the  fate  mainly  lies  in  the  quantity  and  quality  of 
the  food.  Royal  diet,  and  plenty  of  it,  develops  the  reproductive 
organs  of  the  future  queens  ;  sparser  and  ])lainer  food  retards 
the  sexuality  of  the  future  workers,  in  which  reproductive  organs 
do  not  develop.     Up  to  a  certain  point,  the  nurse   bees  can 


THE    DETERMINATION  OF    SEX. 


43 


determine  the  future  destiny  of  their  charge  by  changing  the 
diet,  and  this  in  some  cases  is  certainly  done.  If  a  larva  on 
the  way  to  become  a  worker  receive  by  chance  some  crumbs 
from  the  royal  superfluity,  the  reproductive  function  may  develop, 
and  what  are  called  "fertile  workers,"  to  a  certain  degree  above 
the  average  abortiveness,  result;  or,  by  direct  intention,  a  worker 
grub  may  be  reared  into  a  queen  bee. 

The  following  table,  after  a  recent  analysis  by  A.  von  Planta,  shows  the 
differences  of  diet  as  far  as  solids  are  concerned.  For  queens  69.38  per 
cent.,  for  drones  72.75  per  cent.,  and  for  workers  71.63  per  cent,  is  water. 


Solids. 

Queens. 

Drones. 
I  to  4  clays. 

Drones.        '      ,,,     , 
After  4  days,  j      ^\  orkers. 

Nitrogenous 

Fatty 

Glucose   . . 

Ashes       

45-14 

13-55 

20.39 

4.06 

55-91 
11.90 

9-57 

31-67 
4-74 

38-49 
2.02 

51.21 

6.84 

27.65 

From  the  above,  it  is  seen  that  the  queen  larvce  get  a  (juantity  of  fatty 
material  double  that  given  to  the  workers.  The  drones  at  first  receive  a 
large  percentage  of  nitrogenous  material,  but  this  soon  falls  below  the 
share  which  workers  and  queens  obtain.  The  fatty  material,  at  first 
large,  soon  falls  to  about  a  third  of  that  given  to  the  queens.  Hence  the 
percentage  of  glucose,  except  at  first,  is  so  much  larger  than  in  the  other 
two  cases. 

It  is  not  necessary,  however,  to  go  into  details  to  see  the 
importance  of  the  main  point,  that  differences  of  nutrition,  in 
great  part  at  least,  determine  the  all-important  distinctions 
between  the  development  and  retardation  of  femaleness.  Nor 
are  there  many  facts  more  significant  than  this  simple  and  well- 
known  one,  that  within  the  first  eight  days  of  larval  life,  the 
addition  of  a  little  food  will  determine  the  striking  structural 
and  functional  differences  between  worker  and  queen. 

Eimer  has  drawn  attention  to  the  interesting  correlation  ex- 
hibited in  the  fact  that  a  larva  destined  to  become  a  worker, 
but  converted  into  a  queen,  attains  with  the  increased  sexuality 
all  the  little  structural  and  psychological  differences  which 
otherwise  distinguish  a  queen.  Regarding  fertilisation  as  a  sort 
of  nutrition,  he  considers  drones,  workers,  and  queens  as  three 
terms  of  a  series,  and  the  same  view  is  suggested  by  Rolph. 
Eimer  recalls  some  interesting  corroborations  from  humble  bees. 
There  the  queen  mother,  awakened  from  her  winter  sleep  by 
the  spring  sun,  makes  a  nest,  collects  food,  and  lays  her  first 


44 


THE    EVOLUTION    OF    SEX. 


brood.  These  are  not  too  abundantly  supplied  with  nourish- 
ment, the  queen  having  much  upon  her  shoulders  ;  they  develop 
into  small  females,  workers  in  a  sense,  but  yet  fertile,  though 
only  to  the  extent  of  producing  drones.  By-and-by  a  second 
brood  of  workers  is  born ;  these  have  the  advantage  of  the 
existence  of  elder  sisters,  are  more  abundantly  nourished,  and 
develop  into  large  females.  Still,  like  the  first  brood,  they  pro- 
duce drones,  though  occasionally  females.  Finally,  with  the 
advantage  of  two  previous  broods  of  small  and  large  females, 
the  future  queens  are  born.  The  above  facts  not  only  afford 
an  interesting  corroboration  of  the  influence  of  nutrition  upon 
sexuality,  but  are  of  importance  as  suggesting  the  origin  of  the 
more  highly  specialised  society  of  the  hive  bee. 

{c)  Von  SiehoUVs  Experi7)ienis. — With  a  somewhat  different  purpose 
than  that  at  present  pursued,  Von  Siebold  made  a  series  of  careful  observa- 
tions on  a  species  of  wasp,  Netnattis  ventricostis.  These  afford,  as  Rolph 
has  noted,  some  valuable  results  in  regard  to  the  determination  of  sex.  In 
this  wasp,  the  fertilised  ova,  unlike  those  of  hive  bees,  develop  into  males 
as  well  as  females;  while  the  unfertilised,  or  parthenogenetic  eggs,  may  pro- 
duce females  in  small  percentage.  From  spring  onwards,  as  warmth  and 
food  both  increased,  Von  Siebold  estimated  the  percentages  of  males  and 
females  in  broods  of  larvae  reared  from  fertilised  ova.  The  results  of  a 
series  of  observations  may  be  condensed  in  a  table  : — 


END  OF  Larval  Phriod 

Percentage  of 

No.  of 

No.  of 

(Pupation). 

Females. 

Females. 

Males. 

15th  June 

14 

19 

136 

July      

77 

66 

66 

July       

269 

579 

215 

August  . . 

340 

End  of  August 

500 

September 

100 

As  Rolph  remarks,  the  results  are  not  altogether  satisfactory  for  the 
present  purpose,  "  l)ut  this  much  is  clear,  that  the  percentage  of  females  in- 
creases from  spring  to  August,  and  then  diminishes.  We  may  conclude 
without  scruple,  that  the  production  of  females  from  fertilised  ova  increases 
with  the  temperature  and  with  the  food  supply  {Assiuiilaiionskistiiiig)^ 
and  decreases  as  these  diminish." 

From  the  work  of  Rolph,  which  is  full  of  a  suggestivencss  which  the 
author  unfortunately  did  not  live  to  elaborate,  we  shall  quote  another 
paragraph  .summing  up  further  experiments  of  Von  Siebold  :  — 

"Not  less  instructive,"  he  says,  "are  the  experiments  with  unfertilised 
ova  (see  Table). 

"This  table  shows  the  same  general  result  as  before.  The  more 
abundant    the    metabolism    {Stofpivcchsel)    and    the    nutrition,    the    greater 


THE    DETERMINATION    OF    SEX. 


45 


tendency  to  the  production  of  females,  which  at  the  beginning  and  at  the 
end  are  wholly  absent.  In  the  above  series  of  experiments,  they  only 
appear  when  the  metabolism  and  the  nutrition  were  so  abundant 
that  the  entire  development  of  the  young  wasps  only  occupied  eighteen  or 


No.  of 

Duration  of  Embryonic 

Sex 

Experiments. 

and  Larval  State. 

II 

21  days 

All  Males. 

1 

1 
1 

12 

19      ,, 

All  Males. 

13 

18      ., 

493  Males.         2  Fema 

es. 

14 

17       „ 

265       „              2          ,, 

15 

17       ,, 

374       „               8 

16 

18      ,, 

168       ,,               I 

17 

24      ,. 

I        ,, 

fewer  days  up  to  the  period  of  pupation."  The  peculiarity  in  this  last  case, 
if  the  experiments  w^ere  correct,  is  that  in  parthenogenesis,  where  the 
production  of  males  is  the  normal  condition,  favourable  environmental 
influences  appear  to  introduce  females. 


Two  Forms  of  a  Common  Plant- Louse  or  Ajjhis. — 
This  figure  equally  well  illustrates  three  different 
things, — a  winged  male  and  a  wingless  female ;  a 
winged  and  a  wingless  parthenogenetic  female ;  a 
winged  se-xual  female  and  an  ordinary  wingless 
parthenogenetic  female. — From  Kessler. 

(d)  Case  of  Aphides. — -One  of  the  most  familiar  illustrations 
of  the  influence  of  nutrition  upon  sex,  is  found  in  the  history 
of  the  plant-hce  or  aphides,  which  is  indeed  full  of  other 
suggestions  in  regard  to  the  whole  theory  of  sex  and  reproduc- 
tion. Details  in  regard  to  these  plant-lice,  which  multiply  so 
rapidly  upon   our  rose-bushes,  fruit-trees,  and  the   like,  differ 


46  THE    EVOLUTION    OF    SEX. 

somewhat  in  the  various  species,  but  the  general  facts  are  re- 
cognised to  be  as  follows.  During  the  summer  months,  with 
favourable  temperature  and  abundant  food,  the  aphides  produce 
parthenogenetically  generation  after  generation  of  females.  The 
advent  of  autumn,  however,  with  its  attendant  cold  and  scarcity 
of  food,  brings  about  the  birth  of  males,  and  the  consequent 
recurrence  of  strictly  sexual  reproduction.  In  the  artificial 
environment  of  a  greenhouse,  equivalent  to  a  perpetual  summer 
of  warmth  and  abundant  food,  the  parthenogenetic  succession 
of  females  has  been  experimentally  observed  for  four  years, — it 
seems  in  fact  to  continue  until  lowering  of  the  temperature  and 
diminution  of  the  food  at  once  re-introduce  males  and  sexual 
reproduction. 

{e)  Butterflies  and  Moths. — Still  keeping  to  insects,  we  may 
note  Mrs  Treat's  interesting  experiment,  that  if  caterpillars  were 
shut  up  and  starved  before  entering  the  chrysalis  state  the 
resultant  butterflies  or  moths  were  males,  while  others  of  the 
same  brood  highly  nourished  came  out  females.  Gentry  too 
has  shown  for  moths,  that  innutritious  or  diseased  food  produced 
males,  and  suggests  this  as  a  partial  explanation  of  the  excess 
of  male  insects  in  autumn,  although  we  suspect  that  tempera- 
ture is  in  this  instance  probably  more  important. 

{f)  Crustaceans.  — In  support  of  the  same  contention,  Rolph 
has  drawn  attention  to  the  following  among  other  facts.  One 
of  the  brine  shrimps  {Arteniia  salind)  resembles  not  a  few^ 
crustaceans  in  the  local  and  periodic  scarcity  or  absence  of 
males,  associated  of  course  with  parthenogenesis.  At  Mar- 
seilles, Rolph  says,  this  artemia  lives  in  especially  favourable 
conditions,  as  its  large  size  plainly  indicates ;  there  it  produces 
only  females.  Where  the  conditions  of  existence  are  less 
prosperous,  it  produces  males  as  well.  "  A  certain  maximum 
of  abundance  and  optimum  of  vital  conditions  in  partheno- 
genetic animals — daphnids  and  aphides,  Apus,  Branchipus, 
Artemia,  and  numerous  other  crustaceans — produce  females  ; 
while  less  favourable  conditions  are  associated  with  the  produc- 
tion of  males."  In  regard,  however,  to  water-fleas  (daphnids),  it 
is  fair  to  notice  that  Rolph's  conclusions  do  not  quite  consist 
with  Weismann's,  who,  with  unique  experience  in  regard  to 
these  curious  little  animals,  is  disinclined  to  allow  the  direct 
influence  of  temperature  and  nutrition  in  the  matter. 

{g)  Ma7nmals. — When  w^e  pass  to  higher  animals,  the  diffi- 
culties of  proving  the  influence  of  nutrition  upon  sex  are  much 


THE    DETERMINATION    OF    SEX.  47 

greater.  Yet  there  are  decisive  observations  which  go  to  increase 
the  cumulative  evidence.  Thus  an  important  experiment  was 
long  ago  made  by  Girou,  who  divided  a  flock  of  three  hundred 
ewes  into  equal  parts,  of  which  the  one-kalf  were  extremely 
well  fed  and  served  by  two  young  rams,  while  the  others  were 
served  by  two  mature  rams  and  kept  poorly  fed.  The  propor- 
tion of  ewe  lambs  in  the  two  cases  was  respectively  sixty  and 
forty  per  cent.  In  spite  of  the  combination  of  two  factors,  the 
experiment  is  certainly  a  cogent  one.  Diising  brings  forward 
further  evidence  in  favour  of  the  same  conclusion,  noting,  for 
instance,  that  it  is  usually  the  heavier  ewes  which  bring  forth 
ewe  lambs.  He  emphasises  the  fact,  that  the  females  having 
a  more  serious  reproductive  sacrifice,  are  more  dependent  on 
variations  of  nutrition  than  males.  Even  in  birds,  as  Stolzmann 
points  out,  there  is  a  much  greater  flow  of  blood  to  the  ovaries 
than  to  testes, — the  demands  are  greater,  and  the  consequences 
therefore  more  serious  if  these  are  not  fulfilled. 

(//)  In  the  human  species,  lastly,  the  influence  of  nutrition, 
though  hard  to  estimate,  is  more  than  hinted  at.  Floss  may 
be  mentioned  as  an  authority  who  has  emphasised  this  factor 
in  homo.  Statistics  seem  to  show,  that  after  an  epidemic  or  a 
war  the  male  births  are  in  a  greater  majority  than  is  usually 
the  case.  Diising  also  points  out  that  females  with  small 
placenta  and  little  menstruation  bear  more  boys,  and  contends 
that  the  number  of  males  varies  with  the  harvests  and  prices. 
In  towns,  and  in  prosperous  families,  there  seem  to  be  more 
females,  while  males  are  more  numerous  in  the  country  and 
among  the  poor. 

(/)  Deterniiiiation  of  Sex  in  Plants. — It  is  at  present  ex- 
tremely difficult  to  come  to  any  very  satisfactory  conclusion  in 
regard  to  the  influence  of  nutrition  upon  the  sex  of  plants. 
The  whole  subject,  as  far  as  its  literature  is  concerned,  has 
been  recently  discussed  by  Heyer,  but  his  survey  is  by  no 
means  a  sanguine  one.  His  conclusions,  in  fact,  seem  to  land 
him  in  a  scepticism  as  to  all  modification  of  the  organism  by 
environmental  influences,  which  we  should  of  course  be  far 
from  sharing.  It  must  be  admitted  that  the  experiments  of 
Girou  (1823),  Haberlandt  (1869),  ^^id  others,  yielded  no  cer- 
tain result;  while  the  conclusions  of  some  others,  are  conflicting 
enough  to  justify  not  indeed  Heyer's  despair,  but  his  present 
caution.  Still  a  few  investigations,  especially  those  of  Meehan 
(1878),  which  are  essentially  corroborated  by  Diising  (1883), 


48 


THE    EVOLUTION    OF    SEX. 


go  to  show,  for  some  cases,  that  abundant  moisture  and  nourish- 
ment do  tend  to  produce  females.  Some  of  Meehan's  points 
are  extremely  instructive.  Thus  old  branches  of  conifers, 
overgrown  and  shaded  by  younger  ones,  produce  only  male 
inflorescence.  Various  botanists,  quoted  by  Heyer,  confirm 
one  another  in  the  observation,  that  prothallia  of  ferns  grown 
in  unfavourable  nutritive  conditions  produce  only  antheridia 
(male  organs),  and  no  archegonia  or  female  organs. 

The  botanical  evidence,  though  by  no  means  very  strong, 
certainly  corroborates  the  general  result  that  good  nourishment 
produces  a  preponderance  of  females.  The  contrast  of  the 
sexes  in  our  common  diaecious  plants  is  here  very  instructive. 
I'aking  for  instance  the  dog-mercury  {Mercurialis  perennis)  of 
any  shady  dell,  or  the  day  lychnis  (Z.  diurna),  so  often  hardly 
less  abundant  on  its  sunnier  slopes,  experiments  are  still 
certainly  wanting   with  regard  to  given  plants,  as  to  what  cir- 


Male  and  Female  Flowers  of  Pink  Campion  {Lychnis  diurnci). 

cumstances  originally  determined  their  sexual  differences ;  but 
the  fact  of  superior  constitutional  vegetativeness  in  the  females 
is  here  so  peculiarly  obvious,  that  it  can  hardly  fliil  to  arouse 
a  strong  impression,  that  more  or  less  advantageously  nutritive 
conditions,  whether  of  the  embryo  or  of  the  seedling,  are  suffi- 
cient to  account  for  the  differences  of  sex. 

§  2.  Influence  of  Temperature. — In  this  connection  not  a 
few  writers  have  referred  to  an  observation  by  Knight,  which, 
from  its  comparatively  ancient  date,  perhaps  deserves  to  be 
recorded  in   his  own  words,  if  only  to  show  the  necessity  of 


THE    DETERMINATION    OF    SEX.  49 

caution  in  such  matters.  A  water-melon  was  grown  in  a  heated 
glass-house,  where  the  temperature  sometimes  rose  on  warm 
days  to  iio°  Fahr.  "The  plant  grew  with  equal  health  and 
luxuriance,  and  afforded  a  most  abundant  blossom ;  but  all  its 
flowers  were  male.  This  result  did  not  in  any  degree  surprise 
me,  for  I  had  many  years  previously  succeeded,  by  long  con- 
tinued very  low  temperature,  in  making  cucumber  plants 
produce  female  flowers  only ;  and  I  entertain  but  little  doubt 
that  the  same  fruit  stalks  might  be  made,  in  this  and  the 
preceding  species,  to  support  either  male  or  female  flowers  in 
obedience  to  external  causes." 

This  experiment  was  obviously  more  sanguine  than  satis- 
factory. Heyer  justly  points  out  that  of  the  water-melon  only 
a  single  plant  was  taken.  Furthermore,  he  says,  the  water- 
melon in  nature  usually  bears  only  female  flowers  on  the  apices 
of  the  older  twigs,  and  may  bear  only  a  minimum  number  of 
these.  Knight's  observations  on  cucumbers  are  also  open  to 
serious  objections,  and  were  too  scanty  to  prove  anything. 

Meehan  finds  that  the  male  plants  of  hazel  grow  more 
actively  in  heat  than  the  female ;  and  Ascherson  has  made  the 
interesting  observation,  that  the  water-soldier  {Strafioies  aloides) 
bears  only  female  flowers  north  of  52°  lat.,  and  from  50°  south- 
wards only  male  ones. 

In  the  human  species,  Diising  and  others  have  noted  that 
more  males  are  born  during  the  colder  months;  and  Schlechter 
has  reached  the  same  results  from  observations  upon  horses. 
The  temperature  of  the  time,  not  of  birth  but  of  sex  determina- 
tion, must  of  course  be  noted ;  nor  must  it  be  forgotten  that 
temperature  may  have  its  influence  indirectly  through  the 
nutritive  functions. 

§  3.  Summary  of  Factors. — If  we  now  sum  up  the  case,  it 
must  first  be  recognised  that  a  number  of  factors  co-operate  in 
the  determination  of  sex;  but  that  the  most  important  of  these, 
with  increasing  penetration  of  analysis,  may  be  more  and  more 
resolved  into  plus  or  minus  nutrition,  operating  upon  parent, 
sex  elements,  embryo,  and  in  some  cases  larvae, 

{a)  Starting  with  the  parent  organisms  themselves,  we  find 
this  general  conclusion  most  probable, — that  adverse  circum- 
stances, especially  of  nutrition,  but  also  including  age  and  the 
like,  tend  to  the  production  of  males,  the  reverse  conditions 
favouring  females. 

ib)  As   to  the  reproductive  elements,  a  highly   nourished 

D 


50  THE    EVOLUTION    OF    SEX. 

ovum,  compared  with  one  less  favourably  conditioned,  in  every 
probability  will  tend  to  a  female  rather  than  to  a  male  develop- 
ment. Fertilisation,  when  the  ovum  is  fresh  and  vigorous, 
before  waste  has  begun  to  set  in,  will  corroborate  the  same 
tendency. 

[c)  Then  if  we  accept  Sutton's  opinion  as  to  a  transitory 
hermaphrodite  period  in  most  animals,  from  which  the  transition 
to  unisexuality  is  effected  by  the  hypertrophy  of  the  female  side 
or  preponderance  of  the  male  in  respective  cases,  the  vast 
importance  of  early  environmental  influences  must  be  allowed. 
The  longer  the  period  of  sexual  indifference  (though  this  term 
be  an  objectionable  one)  continues,  the  more  important  must 
be  those  outside  factors,  whether  directly  operative  or  indirectly 
through  the  parent.  Here  again,  then,  favourable  conditions 
of  nutrition,  temperature,  and  the  like,  tend  towards  the  pro- 
duction of  females,  the  reverse  increase  the  probability  of  male 
preponderance. 

The  general  conclusion,  then,  more  or  less  clearly  grasped 
by  numerous  investigators,  is  that  favourable  nutritive  con- 
ditions tend  to  produce  females,  and  unfavourable  conditions 
males. 

§  4.  Let  us  express  this,  however,  in  more  precise  language. 
Such  conditions  as  deficient  or  abnormal  food,  high  temperature, 
deficient  light,  moisture,  and  the  like,  are  obviously  such  as 
would  tend  to  induce  a  preponderance  of  waste  over  repair, — 
a  katabolic  habit  of  body, — and  these  conditions  tend  to  result 
in  the  production  of  7?iales.  Similarly,  the  opposed  set  of 
factors,  such  as  abundant  and  rich  nutrition,  abundant  light 
and  moisture,  favour  constructive  processes,  i.e.,  make  for  an 
anabolic  habit,  and  these  conditions  result  in  the  production  of 
females.  With  some  element  of  uncertainty,  we  may  also 
include  the  influence  of  the  age  and  physiological  prime  of 
either  sex,  and  of  the  period  of  fertilisation.  But  the  general 
conclusion  is  tolerably  secure, — that  in  the  determination  of 
sex,  influences  inducing  katabolism  tend  to  result  in  production 
of  males,  as  those  favouring  anabolism  similarly  increase  the 
probability  of  females, 

§  5.  This  is  not  all,  however;  the  above  conclusion  is  indeed 

valuable,  but  it  acquires  a  deeper  significance  when  we  take  it 

in  connection  with  the  result  of  a  previous  chapter.     There  it 

was  seen,  as  the  conclusion  of  an  independent  induction,  that 

the  males  were  forms  of  smaller  size,  more  active  habit,  higher 


THE    DETERMINATION    OF    SEX.  5 1 

temperature,  shorter  life,  Sue.  ;  and  that  the  females  were  the 
larger,  more  passive,  vegetative,  and  conservative  forms. 
Theories  of  "  inherent  "  maleness  or  femaleness  were  rejected, 
since  practically  merely  verbal ;  more  accurately,  however, 
they  have  been  interpreted  and  replaced  by  a  more  material 
conception,  which  finds  the  bias  of  the  whole  life,  the 
resultant  of  its  total  activities,  to  be  a  predominance  of  the 
protoplasmic  processes  either  on  the  side  of  disruption  or 
construction.  This  conclusion  has  still  to  receive  cumulative 
proof,  but  one  large  piece  of  evidence  is  now  forthcoming,  that, 
namely,  of  the  present  chapter.  If  influences  favouring  kata- 
bolism  make  for  the  production  of  males,  and  if  anabolic 
conditions  favour  females,  then  w^e  are  strengthened  in  our 
previous  conclusion,  that  the  male  is  the  outcome  of  pre- 
dominant katabolism,  and  the  female  of  equally  emphatic 
anabolism. 

§  6.  Weis?fiafi?i''s  Theory  of  Heredity. — In  thinking  of  the 
environment  as  a  factor  determining  the  sex,  it  is  impossible  to 
ignore  that  such  facts  as  we  have  noted  above  have  some 
bearing  upon  the  problem  of  heredity.  Much  of  the  recent 
progress  in  the  elucidation  of  the  facts  of  inheritance  has  been 
due  to  Weismann,  who,  in  his  theory  of  ihe  continuity  of  the 
germ-plasma,  has  restated  the  very  important  and  fundamental 
conception  of  a  continuity  between  the  reproductive  elements 
of  one  generation  and  those  of  the  next.  To  this  restatement 
we  shall  afterwards  have  to  refer ;  it  is  with  another  position, 
not  peculiar  to,  but  emphasized  by  the  same  authority,  that  we 
have  here  to  do,  viz.,  with  his  denial  of  the  inheritance  of 
individually  acquired  characters.  Any  new  character  exhibited 
by  an  organism  may  arise  in  one  of  two  ways,  w^hich  it  is  easy 
enough  to  distinguish  theoretically  ; — it  may  be  an  outcrop  of 
some  property  inherent  in  the  fertilised  egg-cell,  that  is,  it  may 
have  a  constitutional  or  germinal  origin  ;  but,  on  the  other 
hand,  it  may  be  impressed  upon  the  individual  organism  by  the 
environment,  or  acquired  in  the  course  of  its  functioning,  that 
is,  it  may  have  a  functional  or  environmental  origin.  Thus  an 
increase  of  calcareous  matter  in  an  animal  might  well  be  wholly 
of  constitutional  origin ;  but  a  change  to  a  new  diet,  or  to  a  new 
medium,  might  be  followed  by  modifications  arising,  in  one 
sense,  from  without.  But  all  such  functional  and  environ- 
mental variations  are,  according  to  Weismann,  restricted  to  the 
individual  organism  ;  they  are  not  transmissible. 


52  THE    EVOLUTION    OF    SEX. 

And  why  not  ?  This  denial  of  the  inheritance  of  dints  from 
without,  and  of  acquired  habits  other  than  constitutional,  can 
be  no  mere  optimism  on  Weismann's  part.  It  is,  he  maintains, 
a  scientific  scepticism,  based  on  the  one  hand  on  the  absence 
of  data  demonstrating  what  we  may  still  call  the  current  belief, 
and  on  the  other  hand  on  the  improbability  of  changes  pro- 
duced as  above  explained  reacting  from  the  "  body  "  on  the 
reproductive  cells.  If  such  a  reaction  do  not  occur,  Weismann's 
position  is  secure ;  and  though  in  a  system  saturated  with 
alcohol,  or  transferred  to  a  new  climate,  the  reproductive  cells 
may  vary  alotig  with  the  body,  no  modification  of  nerve  or 
muscle  can,  as  such,  be  transmitted  in  inheritance.  In  short, 
the  reproductive  protoplasm  must  be  in  a  sense  insulated,  and 
leads  a  charmed  life  away  from  external  disturbance. 

This  view,  supported  as  it  is  by  many  authorites,  is  obviously 
of  the  utmost  importance,  both  for  the  general  theory  of  evolu- 
tion, and  for  such  practical  problems  as  those  of  the  pathologist 
and  the  teacher.  Its  full  consideration  is  here  impossible, 
involving  matter  enough  for  a  special  treatise  on  heredity. 
The  difficulty  of  any  yea  or  nay  lies  in  the  relative  scarcity  of 
experimental  data,  in  the  divergence  of  opinion  as  to  the 
pathological  evidence,  and  very  largely  in  the  difficulty  of 
applying  our  logical  or  anatomical  distinctions  to  the  intricate 
facts  of  nature.  Thus  the  distinction  between  "acquired,"  and 
germinal  or  constitutional,  is  easily  made  on  paper,  but  is 
difficult  in  actual  practice ;  nor  is  the  line  between  a  variation 
of  the  reproductive  cells,  along  with  the  body,  and  one  produced 
by  the  body,  readily  drawn  in  concrete  cases. 

One  criticism  is  suggested  by  the  present  chapter.  The 
assumed  insulation  or  separateness  of  the  reproductive  elements 
from  the  general  life  of  the  body,  how  far  is  this  real  ?  In  view  of 
the  genuine  unity  of  the  organism,  a  charmed  Hfe  of  one  of  the 
systems  seems  to  some  a  "veritable  physiological  miracle;"  and 
therefore  we  point  to  such  a  case  as  Yung's  tadpoles,  where  an 
outside  influence  of  nutrition  saturated  through  the  organism 
and  did  affect  the  reproductive  elements,  not  indeed  to  the 
degree  of  altering  any  structural  feature  of  the  species,  but  yet 
to  the  extent  of  altering  the  natural  numerical  proportions  of 
the  sexes. 


THE    DETERMINATION    OF    SEX.  53 


SUMMARY 


1.  Nutrition  is  one  of  the  most  important  factors  in  determining  sex.  In 
illustration,  note  {a)  the  experiments  of  Yung,  which  raised  the  percentage 
of  females  from  56  to  92  by  good  feeding  ;  (/')  the  case  of  bees,  where  the 
differences  between  queen  and  worker  well  illustrate  the  enormous  results 
of  a  slight  nutritive  advantage  ;  also  the  case  of  humble-bees,  with  three 
successive  broods  increasing  in  nutritive  prosperity  and  in  femaleness  ;  {c) 
Von  Siebold's  experiments  with  a  wasp,  which  showed  most  females  in 
favoural)le  conditions ;  {(/)  Aphides,  in  prosperity  of  summer,  yield  a 
succession  of  parthenogenetic  females,  in  cold  and  scarcity  of  autumn  males 
return  ;  {e)  starved  caterpillars  of  moths  and  butterflies  become  males  ;  (/) 
Rolph's  observations  on  crustaceans;  {g)  also  the  facts  noted  by  Girou, 
DUsing,  and  others,  on  the  influence  of  good  nourishment  of  mammalian 
mothers  in  favouring  female  offspring  ;  (//)  the  hints  of  the  same  results 
in  the  human  species  ;  (/)  the  various  observations  in  regard  to  plants  which 
favour  the  same  general  conclusion. 

2.  As  to  the  influence  of  temperature,  favourable  conditions  again  lend 
to  femaleness  of  oflspring,  extremes  to  males. 

3.  These  factors  are  now  added  up, — [a)  the  nutrition,  age,  &c. ,  of 
parents  ;  {/>)  the  condition  of  the  sex  elements  ;  {c)  the  environment  of 
embryo. 

4.  The  generalisation  is  thus  reached, — anabolic  conditions  favour 
preponderance  of  females,  katabolic  conditions  tend  to  produce  males. 

5.  But  females  have  been  already  seen  to  be  more  anabolic,  and  males 
more  katabolic.     This  view  of  sex  is  therefore  confirmed. 

6.  How  does  Weismann  explain  the  determination  of  sex,  which  illus- 
trates an  outside  influence  penetrating  to  the  reproductive  cells  ? 


LITERATURE. 

See   works   mentioned    in   Chapter    III.,   especially  those  of  Diising, 
Geddes   (article  Sex,    Ency.    Brit.),    Hensen,  and   Sutton;    also  those   of 
Eimer,  Geddes,  and  Rolph  in  Chapter  II. 
DiJSlNG,  C— As  before  ;  also,  Die  experimentelle  Priifung  der  Theorie  von 

der    Regulirung    des     Geschlechtsverhaltnisses.       Jen.     Zeitschr.    f. 

Naturwiss.  XIV.,  Supplement,  1885. 
Heyer,   F. — Untersuchungen   iiber  das  Verhaltniss  des  Gcschlechtes  bei 

einhiiusigen   und  zweihausigen   Pflanzen,  unter   Beriicksichtigung  des 

Geschlechtsverhaltnisses   bei  den   Thieren   und    den  Menschen,  Ber. 

landwirthschaftl.  Inst.  Halle.  V.  1884,  pp.  1-152. 
Meehan,  T. — Relation  of  Heat  to  the  Sexes  of  Flowers.     Proc.   Acad. 

Nat.  Science,  Philadelphia  (1884),  pp.  111-117. 
Semper,  C. — The  Natural  Conditions  of  Existence  as  they  Affect  Animal 

Life.     Internat.  Science  Series,  London,  1881. 
Thomson,  J.  A. — Synthetic  Summary  of  the  Influence  of  the  Environment 

upon   the    Organism.       Proc.    Roy.    Phys.    Soc.    Edin.,_  IX.    (1888), 

pp.  446-499.     (Supplementary  to  Semper's  work,  with  bibliography.) 
The  History  and   Theory  of  Heredity.     Proc.    Roy.   Soc.   Edin., 

1889,  pp.  91-116,  with  bibliography. 


54  I'HE  p:volution  of  sex. 

Weismann,  a. — Die  Continuil'at  des  Keimplasmas  als  Grundlage  einer 
Theorie  der  Vererbung,  Jena,  1885  ;  and  numerous  other  papers, 
now  translated,  in  i  vol.  —  Essays  upon  Heredity  and  Kindred 
Biological  Problems,  authorised  translation,  edited  by  E.  B.  Poulton, 
S.  vSchonland,  and  A.  E.  Shipley,  8vo.     Oxford,  1889. 

WiLCKENS,  M. — Unteisuchungen  liber  das  Geschlechtsverhaltniss  und  die 
Ursachen  der  Geschlechtsbildung  in  Haustieren.  Biol.  Centralblt. 
VI.  (1886),  pp.  503-510;   Landworlh,  J.  B.,  XV.,  pp.  607-610. 

Yung,  E. — Contributions  a  I'Histoire  de  I'lnfluence  des  milieux  Physiques 
sur  les  Etres  Vivants.  Arch.  Zool.  Exper.,  VII.  (1878),  pp.  251-282  ; 
{1883),   pp.  31-55  ;  Arch.  Sci.   Phys.  Nat.,  XIV.  (1885),  pp.  502-522, 

&.C.,   &.C. 


BOOK     II. 

ANALYSIS   OF    SEX— ORGANS, 
TISSUES,  CELLS. 


CHAPTER    V. 

Sexual  Organs  and  Tissues. 

IT  is  the  object  of  this  portion  of  the  book  to  continue  the 
analysis  of  sexual  characters,  but  now  in  a  deeper  way, 
reviewing  successively  the  organs,  tissues,  and  cells  concerned 
in  sexual  reproduction.  The  essential  and  auxiliary  organs  of 
the  two  sexes,  the  frequent  combination  of  these  in  hermaphro- 
dite plants  and  animals,  the  sex-cells  both  male  and  female, 
will  be  discussed  in  order.  This  survey  will  be  for  the  most 
part  structural  or  morphological ;  the  special  physiology  of  sexual 
union  and  of  fertilisation  will  be  discussed  at  a  later  stage. 

^  I.  Essetitial  Sexual  Organs  of  Animals. — It  is  now  a  well 
established  fact  that  nmong  the  ciliated  infusorians,  which 
swarm  especially  in  stagnant  waters,  a  process  occurs  which 
cannot  but  be  described  as  in  part  sexual  reproduction.  Two 
individuals,  to  all  appearance  alike  be  it  noted,  become  tempor- 
arily associated,  and  interchange  some  of  the  elements  of  their 
accessory  nuclear  bodies.  This  process  of  fertilisation  is 
essential  to  the  continued  vigour  of  the  species,  and  will  be 
afterwards  described  at  length.  Such  a  very  simple  form  of 
sexual  union  differs  from  what  occurs  in  higher  animals,  in  two 
conspicuous  respects, — {a)  the  organisms  are  apparently  quite 
similar  in  form  and  structure;  {b)  they  are  unicellular,  and 
thus  there  is  no  distinction  between  "  body  "  and  reproductive 
cells.  What  is  fertilised  by  the  mutual  exchange  in  those 
infusorians  is,  roughly  speaking,  the  entire  animal,  for  the 
whole  is  but  a  unit  mass  of  living  matter. 

Among  the  protozoa,  however,  loose  colonies  of  cells  occur, 
which  bridge  the  gulf  between  unicellular  and  multicellular 
animals.  In  these  we  find  the  first  indications  of  the  after- 
wards conspicuous  difference  between  "body"  and  reproductive 
cells.  From  these  loose  colonies,  certain  of  the  units  are  set 
adrift,  and  meeting  with  others  more  or  less  like  themselves 
fuse  to  form  a  double  cell,  virtually  a  fertilised  ovum,   from 


58 


THE    EVOLUTION    OF    SEX. 


which  by  continuous  division  a  fresh  colony  is  then  developed. 
In  these  transition  forms  there  are  thus  reproductive  cells  of 
slight  distinctness,  but  as  yet  obviously  no  sexual  organs. 


Volvox,  a  loose  colony  of  cells,  with  some  set  apart  for  reproduction,  after  Kirchner. 

When  we  pass  to  the  sponges,  we  find  colonies  consisting  of 
myriads  of  cells,  among  which  there  is  a  considerable  division  of 
labour.  An  outer  layer  (or  ectoderm)  usually  consisting  of  much 
subordinated  cells,  an  inner  layer  (or  endoderm)  of  predominantly 
active  and  well-nourished  cells,  a  middle  layer  of  heterogeneous 
constituents,  can  always  be  distinguished.  Every  average  infu- 
sorian  is  as  good  as  its  neighbours,  so  far  as  reproduction  of  new 
individuals  by  division  is  concerned ;  in  the  colonial  protozoa, 
the  units  that  are  set  adrift  are  very  little  different  from  their 
fellows  that  remain  behind ;  but  this  ceases  to  be  true  when  we 
pass  to  colonies  where  considerable  division  of  labour  has  been 
established.  It  is  certainly  true  that  even  a  tiny  fragment  of 
sponge,  cut  off  from  the  larger  mass,  may,  if  it  contain  sufficient 
samples  of  the  body,  and  if  the  conditions  be  favourable,  repro- 
duce a  new  individual.  Cultivators  of  bath  sponges  habitually 
take  advantage   of  this  fact.     But  the    sponge   starts  its  new 


SEXUAL    ORGANS    AND    TISSUES.  59 

colonies  for  itself  usually  in  quite  a  different  way,  namely,  by 
the  process  of  sexual  reproduction.  Among  the  cells  of  the 
middle  stratum  of  the  sponge  body  certain  well-nourished 
passive  cells  appear.  These  are  the  ova,  at  first  very  like,  but 
eventually  well  marked  from  the  other  constituent  units  of  the 
layer.  Besides  these  there  are  other  cells,  either  in  the  same 
sponge  or  in  another,  which  exhibit  very  different  characters. 
Instead  of  growing  large  and  rich  in  reserve  material  like  the 
egg-cells  or  ova,  they  divide  repeatedly  into  clusters  of  infini- 
tesimal cells,  and  form  in  so  doing  the  male  elements  or 
spermatozoa.  The  male  and  female  cells  meet  one  another, 
they  form  a  fertilised  ovum ;  the  result  is  continued  division  of 
the  latter  till  a  new  sponge  is  built  up.  Here  then  there  are 
special  reproductive  cells,  quite  distinct  from  those  of  the 
"body";  and  here,  furthermore,  these  reproductive  cells  are 
markedly  contrasted  as  male  and  female  elements.  As  yet, 
however,  there  are  no  sexual  organs. 

Passing  to  the  next  class,  the  stinging  animals  or  coelenter- 
ates,  we  find  in  one  of  the  simplest  and  most  familiar  of  these, 
the  common  fresh-water  hydra,  a  good  illustration  of  primitive 
sexual  organs.  As  in  sponges,  a  cut-off  fragment  of  the  body, 
if  sufficient  samples  of  the  different  component  cells  are  in- 
cluded, is  able  to  reconstitute  the  whole.  But  no  one  body-cell 
has  of  course  any  such  power  ;  this  is  possible  for  the  fertilised 
ovum  alone.  Now  this  ovum  occurs,  not  anywhere  within  a 
given  layer  as  in  sponges,  but  always  near  one  spot  on  the  body. 
Towards  the  base  of  the  tube  a  protuberance  of  outer  layer 
cells  is  developed.  I'his  forms  a  rudimentary  ovary,  or  female 
organ.  It  has  this  peculiarity,  not  however  unique,  that  while 
the  organ  consists  of  not  a  few  cells,  only  one  of  these  becomes 
an  ovum.  A.  similar  protrusion,  or  more  than  one,  often  at  the 
same  time  and  on  the  same  animal,  may  be  recognised  further 
up  the  tube,  nearer  the  tentacles  of  the  hydra.  Of  somewhat 
smaller  size,  such  a  superior  protuberance  consists  of  numerous 
small  cells,  most  of  which,  multiplying  by  division,  form  male 
elements  or  spermatozoa.  We  have  here  the  simplest  possible 
male  organ  or  testis. 

More  elaborate  organs  occur  in  the  other  coelenterates, 
complicated  however  by  two  interesting  facts,  which  will  be 
afterwards  discussed.  {a)  Many  of  the  coelenterates  are  well 
known  to  form  elaborate  colonies, — zoophytes,  Portuguese  men- 
of-wnr,  and   the  like.      In  these,  division  of  labour  frequently 


6o  THE    EVOLUTION    OF    SEX. 

goes  further  than  the  setting  apart  of  special  organs.  Entire 
individuals  become  reproductive  "persons"  (as  they  are  techni- 
cally called),  in  contrast  to  the  nutritive  persons  of  the  colony. 
{b)  In  some  of  those  reproductive  individuals,  a  curious 
phenomenon,  known  as  migration  of  cells,  has  been  observed 
by  Weismann  and  others.  The  reproductive  cells,  arising  in 
various  parts  of  the  body,  have  been  shown  to  migrate  in  some 
cases  to  another  part,  where  they  find  final  lodgment  in  more 
or  less  definite  organs.  This  occurrence  is  intimately  associated 
with  "alternation  of  generation,"  and  will  be  afterwards  discussed 
under  that  heading. 

It  is  in  nowise  the  purpose  of  the  present  work  to  describe 
the  details  respecting  the  ovaries  and  testes,  as  they  occur  in 
the  various  classes  of  animals.  It  is  enough  for  our  purpose  to 
have  emphasised  the  fact  of  their  gradual  differentiation,  and  to 
note  that  they  are  almost  always  developed  in  association  with 
the  middle  layer  of  the  body,  and  usually  occupy  a  posterior 
position  on  the  wall  of  the  body-cavity.  The  details  will  be 
found  in  any  standard  work  on  comparative  anatomy,  very  con- 
veniently for  example  in  Prof.  Jeffrey  Bell's  "  Comparative 
Anatomy  and  Physiology,"  London,  1885. 

§  2.  Associated  Dticts. — It  is  only  in  a  few  animals,  like  hydra  and  its 
allies,  that  the  ovaries  and  testes  are  external  organs,  which  have  simply  to 
burst  to  liberate  their  contents.  They  are  usually  of  course  internal,  and  thus 
arises  the  necessity  of  some  means  of  communication  with  the  outside  world. 
In  the  simplest  cases,  the  male  elements  find  their  way  out  to  the  sur- 
rounding medium  without  any  specialised  mode  of  exit.  They  there  meet, 
by  chance  combined  with  physical  attraction  at  short  range,  with  the  ova, 
which  in  the  simplest  cases  again  have  found  their  way  out  in  an  equally 
primitive  fashion.  Thus  in  the  enigmatical  parasitic  mesozoa  (orlhonectids, 
&c.),  liberation  of  the  germs  may  occur  by  perforation  or  by  rupture  of  the 
excessively  simple  bodies.  In  some  of  the  marine  worms  {e.i^.^  Polygoj-dius\ 
the  liberation  of  the  ova  at  least  is  accompanied  by  the  fatal  rupture  of  the 
mother  organism,  a  vivid  instance  of  reproductive  sacrifice.  Even  in  some 
of  the  common  nereids,  the  same  uneconomical  mode  of  liberation  by  rupture 
appears  to  occur.  The  forcible  rupture  may  be  referred  to  pressure  of  the 
relatively  large  mass  of  growing  cells  which  the  ovaries  often  present. 

As  high  up  as  back-boned  animals,  the  absence  of  ducts  may  be  traced. 
Thus  among  the  sea-squirts  or  tunicates,  the  reproductive  organs  are  fre- 
quently ductless,  and  the  same  thing  is  true  of  some  fishes.  The  sex-cells 
burst  into  the  body-cavity,  and  thence  find  their  way  to  the  exterior  by  aper- 
tures. In  most  cases,  where  ducts  are  absent,  fertilisation  of  the  ova  is 
external,  but  this  is  not  necessarily  so.  In  sponges,  for  instance,  fertilisa- 
tion is  almost  always  internal.  Male  elements  are  washed  in  by  the  water- 
currents,  find  their  way  to  the  ova,  and  fertilise  them  /;/  situ.  Almost 
without  exception,  embryo-sponges,  not  ova,  make  their  way  to  the  exterior. 
In  the  higher  animals,  where  definite  ducts  are  present,  alike  for  the  inward 


SEXUAL    ORGANS    AND    TISSUES.  6 1 

passage  of  spermatozoa  and  the  exit  of  ova  or  embryos,  it  ought  further  to 
be  noticed  that  the  ovaries  can  hardly  ever  be  said  to  be  in  direct  connec- 
tion with  their  ducts.  The  ova  usually  burst  from  the  ovary  into  the  body- 
cavity,  whence  they  are  more  or  less  immediately  caught  up  by,  or  forced 
into  the  canals,  by  which  they  pass  outwards.  With  the  testes  it  is  different, 
for  if  ducts  be  present,  they  are  in  direct  connection  with  the  organs. 

It  is  enough  to  state  that  in  the  great  majority  of  cases  ducts  are 
associated  with  the  essential  organs.  Those  of  the  male  serve  for  the  exit 
of  the  spermatozoa,  and  may  be  terminally  modified  as  intromittent  organs. 
Those  of  the  females  serve  either  solely  for  the  emission  of  unfertilised  eggs, 
or  for  the  reception  of  spermatozoa,  and  the  subsequent  exit  of  fertilised 
ova  or  growing  embryos.  In  some  worm-types,  and  in  all  vertebrates, 
from  amphibians  onwards,  the  reproductive  ducts  are  also  in  various  degrees 
associated  with  excretory  functions.  For  an  account  of  the  origin  of  the 
ducts  in  higher  animals,  the  reader  must  be  referred  to  the  embryological 
text-books  of  Balfour  and  Hertwig,  or  most  conveniently  of  Haddon. 
Similarly  for  such  modifications  as  that  of  the  female  duct  ir)to  oviduct  and 
uterus,  reference  must  be  made  to  the  larger  anatomical  works  of  Gegenbaur 
and  Wiedersheim,  or  for  a  briefer  account  to  Parker's  translation  and  edition 
of  Wiedersheim's  smaller  text-book,  and  to  Prof.  Jeffrey  Bell's  work  already 
mentioned. 

§  3.  Yolk-Glatids.—k.'i  we  shall  afterwards  see,  the  ovum  is 
often  furnished  with  a  large  quantity  of  nutrient  material.  This 
serves  as  the  food-capital  for  the  growing  embryo  or  young  larva. 
It  is  obtained  in  various  ways, — from  the  vascular  fluid,  from 
the  sacrifice  of  adjacent  cells,  or  from  special  organs  known  as 
yolk-glands  or  vitellaria.  The  yolk-glands,  as  they  occur  for 
instance  in  some  of  the  lower  worms  (turbellarians,  flukes, 
tapeworms),  are  of  some  general  interest.  They  represent,  as 
Graff  has  shown,  a  degenerate  portion  of  the  ovary,  in  which 
the  cells  have  become  even  more  highly  nutritive  than  ova. 
"The  origin  of  the  yelk-gland,"  Gegenbaur  says,  "is  probably 
to  be  found  in  the  division  of  labour  of  a  primitively  very  large 
ovary."  In  more  technical  language,  yolk-glands  are  hypertro- 
phied  or  hyper-anabolic  portions  of  the  ovary.  Apart  from 
this  nutritive  capital,  the  egg  is  often  equipped  with  envelopes 
or  shells  of  some  sort,  which  may  be  furnished  by  special 
organs,  or  by  the  sacrifice  of  surrounding  cells,  or  by  the  walls 
of  the  ducts  as  the  eggs  pass  out. 

§  4.  Orgafis  Auxiliaiy  to  Impregnation. — In  most  animals 
in  which  internal  fertilisation  of  the  ova  occurs,  there  are  in 
both  sexes  special  structures  auxiliary  to  the  function  of  impreg- 
nation. Thus  the  end  of  the  male  canal  is  commonly  modified 
into  an  intromittent  tube  or  penis,  through  which  the  male 
elements  flow  into  the  female  duct.  In  the  crustaceans  some 
of  the  external  appendages  are  often  modified,  as  in  the  cray- 


62  THE    EVOLUTION    OF    SEX. 

fish,  to  serve  this  purpose,  and  the  same  is  the  case  with  minute 
structures  on  the  posterior  abdomen  of  many  insects.  Some- 
times, as  in  the  snail  {Helix),  which  may  be  taken  as  an  extreme 
type  of  reproductive  speciaHsation,  separate  organs  are  present, 
in  which  the  spermatozoa  are  compacted  into  masses  or  packets, 
known  as  sperm.atophores.  In  most  cuttle-fishes,  these  pass 
from  the  male  ducts  to  one  of  the  "  arms,"  which  thus  laden  is 
occasionally  set  free  bodily  into  the  mantle-cavity  of  the  female, 
where  it  was  of  old  mistaken  for  a  worm,  and  called  Hectocotyhts. 
So  in  some  spiders,  the  palps  near  the  mouth  receive  the  male 
elements,  and  transfer  them  to  the  female.  Special  storing 
receptacles  and  secreting  glands  are  also  very  frequently  in 
association  with  the  male  ducts,  and  there  is  a  long  list  of 
curious  modifications  utilised  in  the  process  of  copulation. 
'I'hus,  male  frogs  have  their  swollen  thumbs,  and  gristly  fishes 
their  "claspers,"  which  are  modified  parts  of  the  hind  limbs, 
and  are  inserted  into  the  cloaca  of  the  female.  The  common 
snails  eject  a  limy  dart  {spiculinn  amoris),  which  appears  to  be 
a  preliminary  excitant  to  copulation. 

So  too,  in  the  female  sex,  the  terminations  of  the  duct  may 
be  modified  for  reception  of  the  male  intromittent  organ,  and 
special  receptacles  may  be  present  for  storing  the  spermatozoa. 
Where  a  single  fertilisation  occurs,  as  in  the  queen  bee,  previous 
to  a  long-continued  egg-laying  period,  the  importance  of  a 
storing  organ  is  oVjvious.  As  the  female  is  usually  more  or  less 
passive  during  copulation,  the  adaptations  for  this  purpose  are 
less  numerous  than  in  the  males.  It  is  interesting  to  notice, 
that  among  amphibians,  where  the  male  often  takes  upon  him- 
self distinctly  maternal  duties,  one  case  is  known  where  the 
female  seems  more  active  than  the  male  during  copulation. 

^  5.  Egg-Laying  Organs. — Cases  where  the  ova  simply  pass 
out  into  the  water,  or  on  to  the  land,  are  of  course  associated 
with  the  absence  of  any  special  organs.  In  a  great  many 
animals,  however,  more  care  is  taken,  and  auxiliary  structures 
are  present  One  of  the  simplest  of  useful  developments  is 
exhibited  l)y  glands,  the  viscid  secretion  of  which  moors  the 
ova,  and  keeps  them  from  being  set  wholly  adrift.  In  insects, 
where  it  is  specially  important  that  the  eggs  should  be  well  con- 
cealed, or  buried  in  conveniently  nutritive  material,  hints  of 
the  ancestral  abdominal  appendages  remain  as  "ovipositors." 
Throughout  the  series  a  great  variety  of  structures  occur  in  this 
connection. 


SEXUAL    ORGANS    AND    TISSUES.  63 

§  6.  Bj'oodiug  afid  Young-Feeding  Organs.  —  From  very 
lowly  animals  onwards,  structures  are  present  which  are  utilised 
in  the  protection  of  the  young  in  their  helpless  stages.  The 
reproductive  buds  of  some  coelenterates  become  true  nurseries ; 
in  one  at  least  of  the  marine  worms  {^Spu'orbis  spirilhitn)^  a 
tentacle  serves  as  a  brood  pouch ;  various  adaptations,  such  as 
tents  of  spines,  or  cavities  in  the  skin,  are  utilised  in  echino- 
derms.  The  young  shelter  under  the  hard  cuticle,  or  among 
the  appendages  of  crustaceans,  in  the  gills  of  bivalves,  and  a 
cuttle-fish  has  been  seen  with  the  eggs  in  its  mouth.  Among 
the  higher  animals,  the  brood-pouch  of  Appendiculai'ia  (one  of 
the  very  lowest  Chordata),  the  pockets  of  not  a  few  fishes,  the 
cavities  on  the  back  of  the  vSurinam  toad,  the  pouches  of  mar- 
supials, are  only  a  few"  instances  amid  a  crowd.  Sometimes, 
especially  in  fishes  and  amphibians, — e.g.^  the  sea-horse, 
W'ith  its  breast-pouch,  and  Rhinodernia  darwinii^  with  its 
enlarged  croaking  sacs, — it  is  the  male  which  undertakes  the 
brooding  office.  When  the  young  are  born  alive,  the  internal 
female  ducts  become  developed  in  this  connection  to  form 
uteri.  The  ovary  appears  to  serve  as  a  womb  in  the  genus 
Girajdinus  among  fishes,  but  it  is  usually  the  median  portion 
of  the  female  duct  which  has  this  function.  In  placental 
mammals,  where  the  young  are  born  at  an  advanced  stage,  and 
where  the  maternal  sacrifice  is  at  its  maximum,  the  uterine 
adaptations  become  more  important  and  complex.  The  organs 
of  lactation  will  be  afterwards  discussed. 


64  THE    EVOLUTION    OF    SEX. 


SUMMARY. 

1.  The  gradual  differentiation  of  essential  sexual  organs  in  animals, — 
isolated  cells,  aggregated  tissues,  definite  organs. 

2.  Associated  male  and  female  ducts  for  the  liberation  of  male-elements, 
fertilisation,  exit  of  ova,  or  birth  of  embryos. 

3.  Yolk-glands,  &c. ,  for  nourishment  and  equipment  of  the  ova. 
Vitellaria  have  been  interpreted  as  degenerate  ovaries. 

4.  Illustrations  of  organs  auxiliary  to  impregnation.  In  the  male, — 
penis,  storing  sacs,  spermatophore-making  organs,  "  claspers. "  Curiosities, 
such  as  the  hectocotylus  of  cuttle-fishes,  and  the  Cupid's  dart  of  snails. 
Adaptations  in  the  female  are  less  frequent,  but  storing  receptacles  for  the 
male-elements  are  common. 

5.  Egg-laying  organs  : — frequency  of  ovipositors. 

6.  Brood-pouches  and  the  like  are  widely  present  in  most  classes  of 
animals. 

LITERATURE. 

Balfour,    F.    M. — A    Treatise    on    Comparative    Embryology.     2   vols. 

London,  1881. 
Bell,   F.  Jeffrey. — Comparative  Anatomy  and   Physiology.      London, 

1885. 
Claus,  C. — Elementary  Text-Book  of  Zoology,  trans,  by  A.   Sedgwick. 

2  vols.     London,  1885. 
Geddes,  p.  —  Op.  cit. 
Gegf.nbaur,   C. — Elements  of  Comparative   Anatomy,    trans,    by  Prof. 

Jeffrey  Bell.      London,  1878. 
H ADDON,  A.  C. — An  Introduction  to  the  Study  of  Embryology.     London, 

1887. 
Hensen,  V.  —  Op.  cit. 
Hertvvig,  O. — Lehrbuch  der  Entwicklungsgeschichte  des  Menschen  und 

der  Wirbelthiere.     Jena,  1888. 
Hatchett  Jackson's  (W.  )  Edition  of  RoUeston's  Forms  of  Animal  Life. 

Oxford,  1888. 
HuxLEV,  T.  H. — Anatomy    of    Vertebrate    and    Invertebrate    Animals. 

London,  1871  and  1877. 
Sachs,  J. — Text-Book  of  Botany,  edited  by  Prof.  Vines.     Second  edition. 

Oxford,  1882.      And  similar  works. 
Lectures  on  the    Physiology  of  Plants,   trans,  by  Prof.    Marshall 

Ward.     Cambridge,  1887. 
Vines,  S.    H. — Vegetable  Reproduction  (Ency.   Brit.).     Lectures  on  the 

Physiology  of  Plants.     Cambridge,  1886. 
Wiedersheim,   R. — Elements    of   the    Comparative  Anatomy  of   Verte- 
brates,   trans.    l)y   Prof.    W.    N.    Parker.     London,    1886.     Also  un- 
abridged work. 


CHAPTER  VI. 

HERMAPHRODITISM. 

§  I.  When  an  organism  combines  within  itself  the  production 
of  both  male  and  female  elements,  it  is  said  to  be  bisexual  or 
hermaphrodite.  This  is  the  case  with  most  flowers,  and  with 
many  lower  animals, — such,  for  instance,  as  earthworms  and 
snails.  It  is  not  desirable  to  extend  the  term,  as  is  sometimes 
done,  to  cases  like  ciliated  infusorians,  where  sex  itself  is  only 
incipient.  Undoubtedly  in  those  Protozoa  recent  researches 
have  distinguished  what  in  loose  analogy  may  be  called  male 
and  female  nuclear  elements,  but  this  primitive  condition  is 
rather  a  state  antecedent  to  sex,  than  a  union  of  sexes  in  one 
organism. 

In  most  phanerogams,  as  every  one  knows,  male  and  female 
organs  occur  on  different  leaves  (stamens  and  carpels)  of  each 
flower.  The  flower  as  a  w^hole,  or  the  entire  plant,  may  then 
be  called  hermaphrodite.  But  as  the  male  and  female  organs 
are  restricted  to  different  leaves,  each  leaf  is  by  itself  unisexual, 
w^hen  compared,  for  instance,  with  the  prothallus  of  a  fern, 
which  bears  on  the  same  small  expansion  both  male  and  female 
organs.  When  stamens  and  carpels  unite  together,  as  in 
orchids,  a  more  intimate  hermaphroditism  is  obviously  developed. 
So  with  animals.  While  the  general  definition  of  hermaph- 
roditism, as  the  union  of  the  two  sexes  in  one  organism,  is 
plain  enough,  the  union  is  exhibited  in  a  great  variety  of  ways 
and  degrees.     Of  these  it  is  necessary  first  to  take  account. 

§  2.  Embryonic  Hermaph7-oditis7n. — Some  animals  are 
hermaphrodite  in  their  young  stages,  but  unisexual  in  adult  life. 
Allusion  has  already  been  made  to  the  case  of  tadpoles,  where 
the  bisexuality  of  youth  occasionally  lingers  into  adult  life. 
According  to  some,  most  higher  animals  pass  through  a  stage 
of  embryonic  hermaphroditism,  but  decisive  proof  of  this  is 
wanting. 


66  THE    EVOLUTION    OF    SEX. 

The  research  of  Laulanie  may  now  be  refened  to  at  greater  length.  As 
the  result  of  observations  on  the  development  of  the  reproductive  organs  in 
the  higher  vertebrates,  and  especially  in  birds,  he  seeks  to  establish  a  strict 
parallelism  between  the  individual,  and  what  he  believes  to  have  been  the 
racial  history.  In  the  chick,  he  distinguishes  three  main  stages  in  the 
development — (i)  germiparity,  (2)  hermaphroditism,  (3)  differentiated 
unisexuality.  These  he  regards  as  recapitulating  the  great  steps  of  the 
historic  evolution.  (l.)  For  the  first  period  of  "germiparity," — from  the 
fourth  to  the  sixth  day, — the  designation,  sexual  neutrality,  or  indifference,  is 
inappropriate,  since  the  "cortical  ovules"  of  the  germinal  epithelium  have 
from  the  first  the  precise  morphological  significance  of  female  elements  or 
ova.  In  the  female,  they  proceed  by  multiplication  to  form  the  ovary  ;  in  the 
male,  they  degenerate.  (2.)  The  period  of  hermaphroditism  begins  with  the 
seventh  day.  In  the  male,  the  male  ovules,  from  which  the  sperms  are  after- 
wards developed,  appear  in  the  central  tissue  ;  but  at  the  same  time  cortical 
or  female  ovules  may  be  seen  persisting.  Similarly,  in  the  developing  ovary 
of  the  female,  the  central  or  medullary  portion,  strictly  separated  by  a  par- 
tition of  connective  tissue  from  the  egg-forming  layer,  contains  a  large 
number  of  medullary  or  male  ovules.  (3.)  This  hermaphroditism  is  of 
short  duration.  The  cortical  or  female  ovules  disappear  from  the  testes  by 
the  eighth  or  ninth  day  ;  and  the  medullary  or  male  ovules  have  by  the 
tenth  day  disappeared  from  the  ovary.  In  regard  to  mammals,  Laulanie 
affirms,  allowing  some  peculiarities,  that  the  same  three  stages  of  germi- 
parity, hermaphroditism,  and  unisexuality  occur. 

Ploss  has  already  been  referred  to  as  another  investigator  who  maintains 
the  existence  of  embryonic  hermaphroditism.  Such  also  is  the  view  held 
by  Professor  Sutton,  who  concludes  that  both  sets  of  organs  are  equally 
developed  up  to  a  definite  period,  and  emphasises  the  consequent  necessity 
for  the  hypertrophy  of  one  sexual  rudiment  over  the  other.  Only  thus  can 
unisexuality  be  established.  It  ought  perhaps  to  be  noted,  that  hyper- 
trophy is  hardly  a  term  strictly  applicable  to  predominance  of  male  over 
female  organs,  since,  in  our  contention,  the  whole  nature  of  male  organs  or 
elements  is  the  physiological  reverse  of  abundant  nutrition. 

§  3.  Casual  or  Abnormal  Herniaphroditisvi.—Yn  many  species  which 
are  normally  unisexual,  a  casual  hermaphrodite  form,  occasionally  presents 
itself.  The  embryonic  equilibrium  or  bisexuality — one  of  the  t\\o  must  in 
a  variable  degree  exist — is  retained  as  an  abnormality  into  adult  life. 
Even  as  far  up  in  the  organic  series  as  birds  and  mammals,  such  casual  and 
yet  true  hermaphrodites  occur.  In  most  cases  at  least  the  result  is  sterility. 
Among  amphibians,  which  abound  in  reproductive  peculiarities,  herma- 
phroditism exceptionally  occurs,  apart  from  the  one  case  {see  helotv)  where 
it  is  known  to  be  constant.  The  common  frog,  so  much  dissected  in  our 
laboratories,  has  supplied  several  good  illustrations.  Thus  Marshall  notes 
that  the  testes  may  be  associated  with  genuine  ova,  or  an  ovary  may  occur 
on  one  side,  and  a  testis  with  an  anterior  ovarian  portion  upon  the  other. 
Bourne  gives  a  case  of  a  frog  with  the  ovary  well  developed  on  the  right 
side,  and  opposite  this  an  ovary  anteriorly  replaced  by  testis.  One  of  the 
toads  {Pelobales  fuscus)  seems  to  be  frequently  hermaphrodite,  the  male 
being  furnished  with  a  rudimentary  ovary  in  front  of  the  testes.  A  similar 
hermaphroditism  is  not  at  all  infref[uent  in  cod,  herring,  mackerel,  and 
many  other  fishes ;  while  slightly  lower  down  in  the  series,  it  occurs 
in  the  hagfish  {A/yxine).     Sometimes  a  fish  is  male  on  one  side,  female 


HERMAPHRODITISM.  67 

on  the  other,  or  male  anteriorly  and  female  posteriorly.  Sir  J.  W. 
Simpson,  in  a  learned  article  on  the  subject,  has  distinguished  cases  of  true 
hermaphroditism  according  to  the  position  of  the  organs,  into  lateral, 
transverse,  and  vertical  or  double.  Among  invertebrates  the  same  has 
been  occasionally  observed,  especially  among  butterflies  where  striking 
differences  in  the  colouring  of  the  wings  on  the  two  sides  have  in  some 
cases  been  found  to  correspond  to  an  internal  co-existence  of  ovary  and 
testis.  The  same  has  been  observed  in  a  lobster,  and  is  probably 
commoner  than  the  recorded  cases  warrant  one  in  asserting.  As  low  down  as 
coelenterates,  casual  hermaphroditism  may  occur,  as  F.  E.  Schulze  showed 
in  one  of  the  medusoids. 

§4.  Partial  Hermaphroditism. — Kx\.  organism  may  be  said  to  be  truly 
hermaphrodite  when  both  male  and  female  organs  are  present,  or  when, 
without  there  being  separate  organs,  both  male  and  female  elements  are 
produced.  It  is  then  both  anatomically  and  physiologically  hermaphro- 
dite, and  of  this,  as  we  shall  see,  there  are  abundant  illustrations  among 
lower  animals.  Snail,  earthworm,  and  leech  are  examples  of  this  hermaph- 
roditism, in  varying  degrees  of  intimacy. 

But,  as  we  have  just  noticed,  a  species  normally  unisexual  may  occasion- 
ally exhibit  hermaphrodite  individuals.  In  these  only  one  of  the  double 
essential  organs  may  be  functional,  or  both  may  be  sterile.  Whether 
physiologically  or  not,  such  animals  are  anatomically  hermaphrodite.  Both 
kinds  of  essential  organs  are  at  least  present. 

To  those  must  now  be  added  a  further  series  of  cases  to  which  the  term 
partial  hermaphroditism  seems  most  applicable.  Only  one  kind  of  sexual 
organ,  ovarj-  or  testis,  is  developed  ;  but  while  one  sex  preponderates,  there 
are  more  or  less  emphatic  hints  of  the  other.  As  the  reproductive  organs 
are  to  be  regarded  as  the  most  important,  but  not  by  any  means  the  sole 
expression  of  the  fundamental  sex-differences,  it  is  impossible  to  separate 
partial  hermaphroditism  by  any  hard  and  fast  line  from  the  above,  and 
from  the  next  set  of  cases  (paragraphs  3  and  5).  Almost  all  cases  of  partial 
hermaphroditism  occur  as  exceptions,  though  a  few  are  constant. 

In  the  higher  animals,  partial  hermaphroditism  is  usually  expressed  in 
the  nature  of  the  reproductive  ducts.  In  this  connection  the  structural 
resemblance  of  the  male  and  female  organs  must  be  once  more  emphasised. 
Even  the  Greeks  had  their  vague  and  fanciful  theories  of  what  we  now  call 
the  homology  of  the  reproductive  organs  and  ducts  in  the  two  sexes. 
Through  the  labours  of  the  anatomists  of  Cuvier's  school,  such  as  his  fellow- 
worker  Geoffroy  St  Hilaire,  and  yet  more  through  more  recent  embryo- 
logical  discoveries,  there  is  now  both  clearness  and  certainty  as  to  the  main 
facts.  The  reproductive  organs  proper,  the  ducts,  and  the  external  parts, 
are  developed  upon  the  same  plan  in  male  and  female.  Thus,  except  in 
the  lowest  vertebrates,  what  serves  as  an  oviduct  in  the  female,  is  equally 
present  in  the  embryo  male,  and  persists  in  the  adult  as  a  more  or  less 
functionless  rudiment.  In  the  same  way,  what  serves  as  the  duct  for  the 
sperms  {vas  deferens)  in  the  male,  is  equally  present  in  the  embryo  female, 
and  persists  in  the  adult  as  a  rudiment,  or  is  diverted  to  some  other  pur- 
pose. This  is  a  perfectly  normal  occurrence,  dependent  upon  the  embryo- 
logical  history  of  the  ducts  in  question.  It  is  necessary,  however,  to  realise 
both  the  primitive  resemblance  and  the  fundamental  unity  of  the  two  sets 
of  organs,  in  order  to  understand  how  partial  hermaphroditism  is  so  fre- 
quent, and  also  to  distinguish  it  from  "  spurious  hermaphroditism,"  where 


68  THE    EVOLUTION    OF    SEX. 

a  merely  superficial  abnormality  or  even  injury  of  the  ducts  in  one  sex 
produces  a  resemblance  to  those  of  the  other. 

We  have  already  mentioned  that  in  the  case  of  twin  calves,  two  females 
may  occur,  and  both  are  then  normal  ;  or  two  normal  twin  calves  may  be 
born  of  opposite  sexes  ;  but  in  the  third  place,  if  both  be  males,  one  of 
these  very  generally  exhibits  the  peculiar  phenomena  of  what  is  called  a 
"free-martin."  In  the  commonest  form  of  this,  partial  hermaphroditism 
is  well  illustrated.  The  essential  organs  are  male,  but  there  is  a  rudiment- 
ary uterus  and  vagina,  and  the  external  organs  are  further  those  of  a  female. 

It  is'necessary  to  note,  that  a  simulation  of  even  this  partial  hermaphro- 
ditism may  result  from  malformation  or  rudimentary  development  of  the 
external  organs.  On  this  subject  we  may  quote  an  acknowledged  authority, 
alike  in  anatomical  and  embryological  matters.  "  From  the  fact,"  Prof. 
O.  Hertwig  remarks,  "  that  the  external  sexual  organs  are  originally  of 
uniform  structure  in  the  two  sexes,  we  can  understand  the  fact  that,  in  a  dis- 
turbance of  the  normal  development,  forms  arise  in  which  it  is  extremely  diffi- 
cult to  decide  whether  we  have  to  deal  with  male  or  female  external  organs. 
These  cases,  in  earlier  times,  were  falsely  interpreted  as  hermaphroditism. 
They  may  have  a  double  origin.  Either  they  are  referable  to  the  fact  that 
in  the  female  sex  the  development  may  proceed  along  the  same  path  as  in 
the  male,  or  to  this,  that  in  the  male  the  normal  development  may  come  at 
an  early  stage  to  a  standstill,  and  lead  to  the  formation  of  structures  which 
resemble  the  female  parts."  In  the  first  case,  he  goes  on  to  say,  there  may 
be  a  simulation  of  a  penis,  and  the  ovaries  may  even  be  shifted  so  as  to 
produce  an  appearance  like  that  of  the  testes  within  their  scrotal  sac.  In 
the  second  case,  the  processes  of  coalescence  which  give  rise  to  the  penis 
may  not  occur,  only  a  rudimentary  organ  is  formed,  and  there  may  even  be 
an  inhibition  of  the  usual  descent  of  the  testes  into  their  sacs. 

Of  this  superficial  hermaphroditism,  really  not  hermaphroditism  at  all, 
there  are  numerous  cases  among  mammals.  But  there  remain  a  large 
number  of  recorded  instances,  where  the  anatomy  of  the  ducts  was  predomi- 
nantly that  of  the  sex  opposite  to  that  indicated  by  the  essential  organs, 
and  where  the  combination  of  the  two  sexes  was  also  expressed  in  external 
configuration  and  even  in  habit.  Amphibians  again  furnish  some  inter- 
esting examples.  Attached  to  the  anterior  end  of  the  testis  in  various 
species  of  toad  [Biifo),  there  is  an  organ  known  as  "  Bidder's,"  which  has 
contents  like  young  ova.  These  do  not,  however,  get  past  the  early  stages, 
and  the  organ  is  quite  different  from  the  more  than  rudimentary  ovary 
which  occurs  constantly  in  the  males  of  Biifo  ciiiereiis  and  some  other 
species.  The  two  may  in  fact  occur  together.  In  the  common  frog, 
dissectors  have  also  recorded  several  cases  of  hermaphroditism  expressed  in 
the  ducts.  Lastly,  it  is  perhaps  not  going  too  far  to  include  here  some 
reference  to  the  curious  "fatty  bodies"  which  occur  in  all  amphibians  at 
the  apex  of  the  reproductive  organs  in  both  sexes.  These  appear  to  nourish 
the  ovary  and  testis,  esi:)ecially  during  hybernation,  and  may  perhaps  be 
associated  with  similar  lymphoid  structures  in  fishes  and  reptiles.  Prof. 
Milnes  Marshall  was  of  opinion  that  the  fatty  bodies  have  resulted  from 
the  degeneration  of  the  anterior  part  of  the  reproductive  gland  while  still 
in  an  indifferent  state  ;  but  Mr  Giles  has  recently  traced  the  history  of  these 
bodies,  and  shown  them  to  result  from  the  degeneration  of  the  anterior  set 
of  excretory  tubules,  the  pronephros. 

Leaving  the  ducts  out  of  account,   we  may  arrange   the 


HERMAPHRODITISM.  69 

important  phenomena  of  hermaphroditism  in  amphibians  in  a 
series  as  follows  : — 

{a)  Embryonic  heniiaphroditism,  demonstrated  as  of  normal  occurrence 

in  frog  tadpoles. 

,,^-r,     ,•  11  1       T.-         f  expressed  in  Bidder's  organ  in  male  toads ; 

(6')  rartialnermaphroditism,  i  ,  /  ,.  •      ^,   ,       r.,     i     ,  \ 

^  '  '■  '  ( (alsoexpressedmvariousstatesottneducts). 

/  \  T-  1   1..  i.  1       T^-         f  normal  in  some  species  oi  Bufo ; 

\c)  1  rue  adult  hermaphroditism,  \  1  •    r  c 

^  '  '^  '  ( casual  in  frogs,  occ. 

Well-developed  ovary,  rudimentary  ovary  or  Bidder's  organ,  and  "  fatty- 
bodies,"  may  be  taken  as  illustrating  the  normal  and  the  pathological  pre- 
ponderance of  anabolic  processes.  Amphibians,  every  one  will  admit, 
are  for  the  most  part  animals  of  distinctly  sluggish  habit  ;  the  natural 
characteristics  of  the  male  sex  may  be  said  to  be  to  some  extent  handi- 
capped, and  curious  instances  are  known  where  the  more  external  functions 
of  the  two  sexes  are  strangely  inverted.  The  male  obstetric  frog  is  not 
alone  in  taking  charge  of  the  ova,  and  the  female  of  one  of  the  efts  behaves 
in  copulation  like  a  male. 

The  list  need  not  be  further  followed ;  it  is  enough  to  note 
the  very  wide  occurrence  of  partial  hermaphroditism.  In  many 
cases,  howTver,  this  takes  an  interesting  form,  by  expressing 
itself  in  the  external  characters.  Forms  occur  in  which  the 
minor  peculiarities  of  the  two  sexes, — colouring,  decorations, 
weapons,  and  the  like, — appear  blended  together,  or  in  which 
the  secondary  sexual  characters  are  at  variance  with  the  internal 
organs.  In  most  cases,  one  is  safe  in  saying  that  there  is  no 
true  internal  hermaphroditism  in  any  degree.  Arrest  of  matu- 
rity or  puberty,  cessation  of  the  reproductive  functions,  removal 
or  disease  of  the  essential  organs,  and  the  like,  may  alter  the 
secondary  sexual  characters  from  female  towards  male,  or,  less 
frequently,  %uce  versa.  A  female  deer  may  develop  a  horn,  or  a 
hen  a  spur,  and  in  such  cases  the  ovaries  are  generally  found  to 
be  diseased.  The  prettiest  cases  of  superficial  hermaphroditism 
occur  among  insects,  especially  among  moths  and  butterflies, 
where  it  often  happens  that  the  wings  on  one  side  are  those  of 
the  male,  on  the  other  those  of  the  female.  Only  the  external 
features  have  been  observed  in  most  cases  ;  but  it  has  been 
shown  by  dissection  that  such  superficial  blending  may  exist 
along  with  internal  unisexuality,  or,  in  a  few  cases,  with  genuine 
internal  hermaphroditism.  A  beautiful  case  of  intimate  blend- 
ing of  superficial  sex  characters  was  lately  shown  to  us  by  Mr 
W.  de  V.  Kane,  of  Kingstown.  A  specimen  of  butterfly 
{Eiichloe  euphenoides)  showed  the  anterior  half  of  the  fore  wings 
and  part  of  the  hind  wings  with  the  characteristic  white  ground 
of  the  female,  while  in  the  posterior  half  of  the  fore  wings  and 


yo  THE    EVOLUTION    OF    SEX. 

on  most  of  the  hind  wings  the  characteristic  sulpiiur  of  the 
male  prevailed.  In  other  minor  ways,  the  characteristics  of  the 
two  sexes,  which  are  well  marked,  were  intimately  Ijlended. 
Similar  cases  are  on  record. 

§  5.  Normal  Adult  Hermaphroditism. —  This  is  rare  among 
the  higher  animals,  but  common  among  the  lower.  On  the 
threshold  of  the  vertebrate  series,  we  find  it  indeed  constant 
among  the  tunicata ;  but  above  these  it  is  only  known  to  occur 
normally  in  two  genera  of  fishes,  and  in  one  genus  of  amphibians. 
"  A  testis  is  constantly  found  imbedded  in  the  wall  of  the  ovary 
in  Chrysophrys  and  Sej-ranus,  and  the  last-named  fish  is  said  to 
be  self-impregnating."  In  some  species  of  male  toad  {e.g.,  Biifo 
cinereus)  a  somewhat  rudimentary  ovary  is  always  present  in 
front  of  the  testes.  All  other  cases  among  vertebrates  are 
either  casual  (par.  3)  or  partial  (par.  4).  Among  invertebrates, 
true  hermaphroditism  is,  however,  of  frequent  normal  occur- 
rence, e.g..,  in  sponges,  cfjelenterates,  worm  types,  and  molluscs. 
It  is  necessary  to  take  a  brief  survey  of  some  of  these. 

(i.)  Sponges.  —As  already  mentioned,  the  sex-cells  of  sponges  start 
sini{)ly  among  the  other  components  of  the  middle  layer  {ntesoglad)  of  the 
body.  It  is  at  least  possible  that  in  any  sponge  they  may  devcU^p  either 
into  ova  or  into  sjierms,  or  into  both,  within  the  same  organism,  according 
to  nutritive  and  other  conditions.  The  facts,  however,  arc  these.  Many 
sponges  are  only  known  in  a  unisexual  state,  while  others  are  genuinely 
hermaphrodite.  But  among  the  latter  it  is  not  uncommon  to  find  {e.g..^  in 
.'^ycandra  raplianns)  that  the  production  of  one  set  of  elements  prepon- 
derates over  the  other,  and  thus  we  have  ht  rmaphrodites  with  a  distinctly 
male  or  female  bias.  In  other  words,  they  arc  verging  towards  unisexuality. 
It  does  happen  in  fact  (<^..^".,  in  Oscarella  lolmlaris)  that  a  species  normally 
hermaphrodite  may  exhibit  unisexual  forms.  It  is  possible,  of  course,  that 
in  such  cases  one  set  of  sexual  elements  may  have  been  wholly  discharged, 
or  may  even  have  been  overlooked  in  observation  ;  but  there  is  no 
improbability  against  the  supposition,  that  a  j)reponderance  of  favourable 
nutritive  conditions  might  induce  a  form  normally  herma])hrodite  to  become 
wholly  female.  This,  as  we  have  seen  above,  is  what  some  believe  to  take 
place  in  the  individual  history  of  higher  forms. 

(2.)  Calcnteraics. — The  members  of  this  class  are  higher,  in  having  the 
production  of  the  sex-cells  more  restricted,  to  definite  regions,  tissues, 
organs,  or  even  "  persons."  The  highly  active  Ctcnophores,  like  Bcrdc,  are 
all  hermaphrodite,  and  that  very  closely.  On  one  side  of  the  meridional 
branches  of  the  alimentary  canal  ova  arise,  on  the  other  side  sperma- 
tozoa. Among  sea-anemones  and  corals  the  hermaphrodite  condition 
apjK-ars  in  a  number  of  cases,  but  is  sometimes  obscured  by  the  fact  that 
the  two  kinds  of  elements  are  produced  at  different  times,  corrcspondmg  to 
different  physiological  rhythms  in  the  life  of  the  organism.  The  genus 
Coralliuni  (the  red  coral  of  commerce)  is  peculiarly  instructive.  The  whole 
colony  may  be  unisexual,   or  one  branch   of  the  colony,  or  only  certain 


HERMAPHRODITISM. 


71 


individuals  on  a  branch,  while  genuine  hermaphrodilism  of  individual 
polyps  also  occurs.  Among  hydrozoa  (zoophytes,  swimming-bells,  jelly- 
fish), hermaphrodilism  is  a  rare  exception,  or,  we  may  almost  say,  rever- 
sion. The  couunon  hydra,  which  is  a  somewhat  degenerate  type,  is 
hermaphrodite,  though  at  the  same  time  individuals  may  be  found  with 
only  ovary  or  only  testes.  Eleutheria  is  also  hermaphrodite,  and  "  abor- 
tive ova  occur  in  the  male  of  GonotJiyrea  loveiii.^^  Sometimes  a  colony  is 
hermaphrodite  {Dicoiyne),  but  the  stems  and  individuals  unisexual.  Some- 
times a  stem  is  hermaphrodite,  but  the  individuals  unisexual  (certain 
sertularians).  Among  jelly-fishes  the  genus  C/uysaora  is  known  lo  be 
hermaphrodite. 

(3.)  "  JVor/iis.'^ — Theconditionof  the  sexual 
organs  varies  enormously  among  the  diverse 
types  lumped  together  under  the  title  of 
"worms"  or  "Vermes."  In  the  lowly  tur- 
bellarians,  all  the  genera  are  hermaphrodite 
except  two,  but,  as  in  many  other  cases,  the 
organs  do  not  reach  maturity  at  the  same  time, 
the  male  preceding.  In  the  related  trematodes 
or  flukes,  hermaphroditism  again  obtains,  with 
one  exception,  or  perhaps  two.  The  certain 
exception  is  the  curious  parasite  Bil/iarzia, 
where  the  male  carries  the  female  about  with 
him  in  a  "  gyncecophoric  canal,"  formed  of 
folds  of  skin.  In  the  adjacent  class  of  cestodes 
or  tapeworms,  all  the  members  are  hermaph- 
rodite. These  three  classes  are  doubtless  re- 
lated, but  it  seems  plausible  to  connect  the 
retention  of  hermaphroditism  with  the  de- 
generacy of  parasitism,  and  also  with  the  rich, 
yet  at  the  same  time  stimulating  nutrition, 
which  may  favour  the  retention  of  double 
sexuality.  The  utility  of  the  hermaphrodite 
state,  if  the  eggs  of  these  animals  are  to  be 
fertilised  and  the  species  maintained,  can 
hardly  be  doubted,  but  this  does  not  explain 
the  facts.  It  is  important  to  notice  too,  that 
self- fertilisation — that  is,  union  of  the  eggs  and 
sperms  of  the  same  organism — has  been  proved 
to  occur  in  several  trematodes,  and  seems  to 
be  almost  universal  in  cestodes.  This  may  be  one  of  the  conditions  of  the 
degeneracy  of  these  parasites,  for  frequent  as  hermaphroditism  is  among 
plants  and  animals,  self-fertilisation  is  extremely  rare. 

Hermaphroditism  is  rare  among  the  free-living  nemerteans,  but  con- 
stant in  the  semi-parasitic  leeches.  The  only  exception  to  separateness  of 
the  sexes  among  threadworms  or  nematodes  is  the  very  curious  case  of  the 
genus  Angiostotmiin.  Here,  in  an  organism  which  is  anatomically  a  female, 
the  reproductive  organ  starts  with  producing  spermatozoa,  which  fertilise 
the  subsequent  ova.  The  animal  is  thus  physiologically  hermaphrodite, 
and  at  the  same  time  self-impregnating.  Approaching  the  higher  annelid 
worms,  we  find  the  primitive  Protodrihis  hermaphrodite  ;  the  earthworms 
are  constantly  so,  but  all  their  marine  relatives  have  the  sexes  separate. 


Bilharzia,  a  parasitic  treinatode, 
in  which  the  male  carries  the 
female  in  a  special  fold  of  skin 
called  the  "  gyncecophoric 
canal." — After  Leuckart. 


72  THE    EVOLUTION    OF    SEX. 

The  genus  Sagitta,  which  stands  by  itself,  is  hermaphrodite  ;  the  same 
condition  is  known  as  a  rarity  among  the  ancient  brachiopods  [Lingtcla), 
but  is  frequent  among  the  colonial  Polyzoa. 

{4.)  Ecliinoderniata. — The  members  of  all  the  echinoderm  class,  except 
one  brittle  star  {Anipliiiira  sqiianmta)  and  one  genus  of  holothurians 
{Synapla),  have  the  sexes  separate. 

(5.)  Arthropods. — Among  crustaceans,  hermaphroditism  is  a  rare  ex- 
ception, though  it  occurs  in  the  majority  of  the  fixed  quiescent  acorn-shells 
and  barnacles  {Cirripedia).  There  it  is  associated  with  the  presence  of 
small  males,  which  Darwin  called  "  complemental."  In  the  Cymothoidre 
(Isopods),  we  have  a  curious  occurrence,  somewhat  like  that  oi Augiostoinuin 
above  noticed.  "The  sexual  organ  of  the  young  animal  is  male,  of  the 
old,  female  in  function."  In  such  cases,  one  must  remember  the  antithesis 
between  the  body  proper  and  the  reproductive  cells.  In  youth  the  demands 
of  the  body  during  growth  are  greater ;  there  is  no  anabolic  surplus  to 
spare,  all  goes  to  increase  the  body.  When  mature  size  is  reached,  and 
both  growth  and  activities  lessened,  there  is  more  likelihood  of  anabolic 
preponderance  in  the  reproductive,  as  opposed  to  the  vegetative,  system. 

Myriopods  and  insects  have  always  separate  sexes,  excluding  of  course 
abnormal  hermaphroditism  among  the  latter.  An  exception  among  arach- 
nids, otherwise  unisexual,  is  found  in  the  degenerate  water-bears  or  sloth- 
animalcules  ( Tardigrada). 

(6.)  Molluscs. — Most  bivalves  are  of  separate  sexes,  but  exceptions 
often  occur — e.g.^  in  common  species  of  oyster,  cockle,  clam,  &c.  In  the 
case  of  the  oyster,  the  familiar  species  {Ostrea  ediilis)  is  hermaphrodite, 
and  a  neighbouring  species  apparently  unisexual.  In  both  cases  the  organs 
are  the  same,  but  in  O.  edulis  the  same  intimate  recesses  of  the  reproduc- 
tive organ  produce  at  one  time  ova,  at  another  time  sperms. 

The  snails,  or  gasteropods,  are  divided  into  two  great  groups,  according 
to  the  twisting  of  their  nerves.  The  one  group  {Strcptoneiird)  have  the  sexes 
separate  ;  the  members  of  the  other  series  {Eiitliyiienra)  are  hermaphrodite. 

The  sea-butterflies,  or  pteropods,  are  hermaphrodite,  but  the  elephant's 
tooth  shells  (vScaphopods)  are  unisexual.  So  in  cuttle-fishes  (Cephalo- 
pods),  the  sexes  are  separate. 

§  6.  Degrees  of  Normal  Herniaplwodiiisni. — From  what  has 
been  already  said,  it  is  evident  that  hermaphroditism  may  be 
more  or  less  intimate.  As  an  entire  plant,  an  Aniiti  is  herma- 
phrodite, with  female  flowers  on  the  better  nourished  lower 
portion,  and  male  flowers  above.  This  may  be  paralleled  by 
the  red  coral,  which  is  sometimes  female  as  regards  one  branch, 
and  male  as  regards  another.  If  we  keep,  however,  to  herma- 
phrodite individuals,  it  is  evident  that  an  orchid,  with  stamens 
and  carpels  united,  is  more  closely  hermaphrodite  than  a  butter- 
cup flower.  So  in  a  leech,  with  the  ovaries  far  forward,  and 
independent  of  the  long  row  of  testes,  the  hermaphroditism  is 
less  intimate  than  in  a  tunicate,  where  the  testes  and  ovary  may 
form  one  mass,  the  male  cells  spreading  over  the  surface  of  the 
ovary.     In  the  same  way,  the  organ  of  a  scallop,  which  exhibits 


HERMAPHRODITISM.  73 

more  or  less  distinct  male  and  female  portions,  is  in  a  state 
of  less  intimate  anatomical  hermaphroditism  than  the  oyster, 
where  the  same  c^ca  of  the  same  organ  fulfil  both  functions  at 
differejit  times. 

This  last  caution  must  be  kept  in  view  throughout.  If  the 
hermaphroditism  be  very  intimate, — that  is,  if  the  seats  of  the 
ovum-  and  sperm-production  be  very  close  to  one  another, — it  is 
not  to  be  expected  that  the  development  of  the  two  kinds  of 
cells  will  go  on  simultaneously.  Such  would,  indeed,  be  a  phy- 
siological impossibility.  Antagonistic  protoplasmic  rhythms 
may  rapidly  alternate,  but  cannot  co-exist.  Whether  the  herma- 
phroditism be  anatomically  intimate  or  no,  there  is  throughout, 
in  varying  degrees,  a  tendency  to  periodicity  in  the  production 
of  male  and  female  elements.  Such  a  want  of  "  time-keeping  " 
between  the  sexes  is  called,  in  botanical  language,  dichogamy, 
and  is  one  of  the  conditions  which  rend^  self-fertilisation  rarely 
possible.  Both  in  plants  and  in  animals,  the  male  function  has 
in  the  majority  of  cases  the  precedence.  Thus  "  protandrous 
dichogamy  "  (stamens  taking  the  lead)  is  very  much  commoner 
than  "protogynous  dichogamy,"  where  the  carpels  are  first  of 
all  matured.  This  agrees  with  the  curious  cases  oi  Angiostomum 
and  CyJiiothoidiE  already  mentioned,  where  the  organ  was  first 
male  and  then  female,  and  indeed  with  at  least  most  cases  among 
closely  hermaphrodite  animals.  A\' here  the  male  organs  are  situ- 
ated in  one  part  of  the  body,  and  the  females  in  another,  there  is 
less  reason  against  the  production  of  sperms  going  on  at  the  same 
time  as  the  production  of  ova.  The  very  physiological  condi- 
tions which  first  determined  the  position  of  the  ovaries  here  and 
the  testes  there,  may  remain  to  render  it  possible  for  the  two 
opposing  functions  to  go  on  at  the  same  time. 

The  common  snail  [Helix)  is  not  only  easily  dissected,  but 
in  the  complexity  of  its  arrangements  is  full  of  interest.  Here, 
not  only  are  ova  and  sperms  produced  within  the  compass  of 
one  small  organ,  but  each  little  corner  of  the  organ  shows 
female  cells  forming  on  the  walls  and  male  cells  in  the  centre. 
It  has  been  justly  suggested  by  Platner  that  the  outer  cells 
are  the  better  nourished ;  they  therefore  naturally  become 
developed  into  anabolic  ova. 

§  7.  Self-Fertilisation. — We  have  noted  above,  that  though 
male  and  female  organs  be  present  in  the  same  organism,  they 
tend  to  become  mature  at  different  times,  and  that  the  more 
the  closer  the  seats  of  formation  of  the  two  kinds  of  elements. 


74  THE    EVOLUTION    OF    SEX. 

It  is  e(|iially  necessary  to  emphasise,  that  though  both  male  and 
female  elements  may  be  produced  in  the  same  plant  or  animal, 
it  is  probably  exceptional  for  the  ovule  to  be  penetrated  by  a 
pollen  cell  from  the  same  flower,  and  it  is  certainly  rare  for  an 
animal  to  fertilise  its  own  ova. 

It  is  believed  by  breeders  of  higher  animals  that  "  close- 
breeding  "  beyond  a  certain  point  is  dangerous  to  the  welfare  of 
the  breed.  The  offspring  tend  to  be  abnormal  or  unhealthy. 
In  view  of  this,  the  rarity  of  self-fertilisation  among  herma- 
phrodites has  been  explained  in  terms  of  the  disadvantage  of 
the  process.  In  reality,  however,  this  is  putting  the  cart  before 
the  horse.  In  hermaphrodites,  we  take  it  that  the  two  kinds  of 
sexual  elements  mature  and  are  liberated  at  different  times,  not 
because  of  any  reaction  of  the  disadvantageousness  of  self- 
fertilisation  on  the  health  of  the  species,  l)ut  simply  because  the 
simultaneous  co-existence  of  opposite  physiological  processes  is 
in  varying  degrees  prohibited.  More  technically,  dichogamy  is 
not  a  subsequent  result  of  the  disadvantage  of  self-fertilisation, 
but  cross-fertilisation  is  the  subsequent  result  of  increasing 
dichogamy. 

Self-fertilisation  does,  however,  occur  as  an  exception  among 
animals, — thus  in  all  probability  in  the  exceptional  fish 
Serranus :  certainly  in  many  parasitic  flukes  or  trematodes ; 
"  commonly,  if  not  universally,"  in  tape-worms  or  cestodes ; 
also  in  the  curious  thread-worm  AngiostoinunK  and  probably 
in  ctenophores,  and  in  some  other  cases.  In  regard  to  some 
cases,  e.g.^  among  hermaphrodite  bivalves  (where  the  sperms 
are  usually  wafted  in  with  the  water),  it  is  impossible  as  yet  to 
say  whether  self-impregnation  does  or  does  not  occur.  Some 
curious,  but  not  very  reliable,  observations  are  on  record  in 
regard  to  self-impregnation  in  casually  hermaphrodite  insects. 

Arguing  from  the  bad  effects  of  close  breeding  among  higher 
animals,  Darwin  and  others  have  called  attention  to  the 
numerous  contrivances  among  plants  which  are  said  to  render 
self-fertilisation  impossible.  It  must  again  be  said,  that  this 
survival  of  a  very  old  way  of  explaining  facts — in  terms  of  their 
final  advantage — is  not  really  a  causal  explanation  at  all.  It 
has  been  pointed  out,  that  in  some  cases  the  pollen  of  a  given 
flower  is  quite  ino})erative  on  the  ovule  of  the  same  flower,  or 
has  the  result  of  producing  weakly  offspring.  Then  there  are 
a  great  variety  of  mechanical  devices,  as  the  result  of  which  it 
is  more    or  less    physically  impossible  for    the    pollen  of  the 


HERMAPHRODITISM.  75 

stamens  to  reach  the  stigmas  of  the  flower,  or  even  to  be  dusted 
upon  them  by  the  unconscious  agency  of  the  intruding  insects. 
Moreover,  as  among  animals,  so  among  plants,  it  is  common 
for  the  male  organs  to  become  mature  before  the  carpels  are 
ready,  or,  in  rarer  cases,  for  the  reverse  to  occur. 

There  is  no  doubt  that  cross  fertilisation  very  generally 
occurs,  and  it  is  physiologically  probable  that  this  is  a  con- 
siderable advantage,  though  less  among  plants  (which  are  so 
very  "female,"  i.e.^  vegetative)  than  among  animals.  But  there 
is  an  increasing  impression  that  both  the  occurrence  of  cross- 
fertilisation,  and  the  necessity  of  it  among  higher  plants,  have 
been  exaggerated  by  the  extreme  Darwinian  school.  One  of 
the  most  thoughtful  and  observant  of  American  botanists,  Mr 
T.  Meehan,  has  raised  a  vigorous  protest  against  the  prevalent 
view.  In  the  Yucca^  or  Adam's  needle,  which  is  regarded  as 
cross-fertilised  by  insects,  he  showed  by  experiment  that  there 
was  in  each  flower  "  no  abhorrence  of  its  own  pollen."  "  Even 
when  fertilised  at  all  by  insects,  I  am  sure  the  fertilisation  is 
from  the  pollen  of  the  same  flower." 

Then  as  to  mechanical  contrivances,  he  says,  "  we  are  told 
that  iris,  campanula,  dandelion,  ox-eye  daisy,  the  garden  pea, 
lobelia,  clover,  and  many  others,  are  so  arranged  that  they 
cannot  fertilise  themselves  without  insect  aid.  I  have  enclosed 
flowers  of  all  these  named  in  fine  gauze  bags,  and  they  produced 
seeds  just  as  well  as  those  exposed." 

We  cannot  here  enter  into  a  full  statement  of  Meehan's 
careful  observations,  but  his  three  main  propositions  well 
deserve  statement  and  due  consideration : — 

1.  Cross-fertilisation  by  insect  agency  does  not  exist  nearly 
to  the  extent  claimed  for  it. 

2.  Where  it  does  exist,  there  is  no  evidence  that  it  is  of  any 
material  benefit  to  the  race,  but  to  the  contrary. 

3.  Difficulties  in  self-fertilisation  result  from  physiological 
disturbances,  that  have  no  relation  to  the  general  welfare  of 
plants  as  species. 

§  8.  Comple?iiental  Males. — When  Mr  Darwin  was  inves- 
tigating barnacles  and  acorn  shells,  in  preparation  for  his 
monograph  on  the  group,  he  discovered  the  remarkable  fact 
that  some  of  the  hermaphrodite  individuals  carried  minute 
males  concealed  under  their  shells.  These  he  regarded  as 
advantageous  accessory  forms,  ensuring  cross-fertilisation  in  the 
hermaphrodites  which  harbour  them.     The  great   majority  of 


76 


THE    EVOLUTION    OF    SEX. 


the  cirripedes  are  hermaphrodite ;  but  among  the  barnacles 
proper, — the  stalked  forms,  which  are  nearer  the  ancestral  type, 
— separate  sexes  sometimes  occur.  On  the  females  of  a  few 
of  these,  pigmy  males,  like  those  found  upon  hermaphrodites, 
also  occur.  These  pigmy  males,  whether  on  females  or  herma- 
phrodites, are  not  only  dwarfish,  but  are  very  often  degenerate, 
sometimes  wanting  (according  to  Darwin)  both  alimentary 
canal  and  thoracic  legs.  Some  of  them,  in  fact,  are  little  more 
than  parasitic  testes. 

(i.)  The  original  state  of  affairs  in  this  case  was  probably  the  ordinary 
crustacean  condition  of  separate  sexes.  (2.)  The  males,  as  in  some  of  the 
"water-fleas"    or   copepods,  tended  to  be  smaller, — smaller  indeed  to  a 


Myzostomata  : — A  hermaphrodite  (i)  and  a  pigmy  male  (2).  —  Itoiii  Naiisen. 


vanishing  point, — while  the  females  became  more  and  more  sluggish,  and 
settled  down.  (3.)  In  the  genera  Alcippe  and  Crypiophiahis^  in  the  species 
Iblaciii/iiningii  and  Scalpelbuii  oniattiiii^  we  find  true  females,  with  attached 


HERMAPHRODITISM.  77 

pigmy  males,  often  several,  leading  a  shabby  existence  as  parasites,  (4.) 
In  other  species  of  Scalpelhim  and  Ibla  the  same  pigmy  males  occur,  but 
attached,  as  we  have  noted,  to  hermaphrodites,  which  in  these  forms  have 
replaced  the  true  females.  (5.)  Lastly,  in  many  genera,  like  Follkipes, 
only  hermaphrodites  occur. 

What  Darwin  did  for  the  cirripedes,  Graff  has  done  for  another  very 
curious  set  of  animals,  the  Myzostomata.  These  are  degenerate  chcetopods 
or  bristle-footed  v/orms,  which  occur  as  outside  parasites  on  sea-lilies 
(crinoids),  on  the  arms  of  which  they  make  curious  galls.  The  majority 
are  hermaphrodite,  but  again  some  species  have  the  sexes  separate,  and 
again  in  a  few  cases  complemental  males  have  been  demonstrated.  If  the 
hermaphrodite  condition  was  here  primitive,  it  persists  in  the  majority  of 
cases ;  thus,  IMyzostonia  glabriirn  is  hermaphrodite,  with  a  minute  com- 
plemental male  ;  AI.  cysticoliim  has  the  sexes  distinct,  but  the  female  is  just 
emerging  from  (or  approaching)  hermaphroditism,  for  it  includes  rudi- 
mentary testes  ;  in  M.  tenuispinuni,  injlator,  murrayii,  there  are  separate 
sexes,  with  the  females  predominating  in  size.  One  conclusion,  at  least, 
is  vividly  suggested  by  these  curious  facts,  the  tendency  of  the  male  form 
to  become  reduced  to  a  vanishing  point. 

§  9.  Conditions  of  Hertnaphroditism. — In  looking  back  over 
the  cases  where  normal  hermaphroditism  occurs,  a  few  general 
conclusions  are  readily  drawn.  Thus  Claus  points  out  that 
hermaphroditism  finds  most  abundant  expression  in  sluggish 
and  fixed  animals.  Flat-worms,  leeches,  earthworms,  tardi- 
grades,  land  snails,  &c.,  well  illustrate  the  first  of  these ;  and 
among  sponges,  sea-anemones,  corals,  polyzoa,  bivalves,  &c.,  we 
find  frequent  illustration  of  the  association  of  fixedness  and 
hermaphroditism.  Most  of  the  tunicates  are  also  fixed,  and  all 
are  hermaphrodite.  Claus  notes  further,  how  in  flukes  and 
tapeworms  hermaphroditism  is  associated  with  isolated  habit  of 
life.  The  isolation,  however,  is  only  sometimes  true,  for  flukes 
may  occur  near  one  another  in  great  numbers ;  and  as  many  as 
ninety  tapeworms  {Bothriocephalus)  have  been  known  to  occur 
at  one  time  in  a  single  host. 

Simon  has  gone  further,  in  insisting  on  the  real  connection 
between  quiescent  and  parasitic  habit  and  the  hermaphrodite 
condition.  In  flukes  and  tapeworms,  leeches,  Myzostomata, 
and  some  cirripedes,  we  find  the  association  of  hermaphroditism 
with  a  more  or  less  intimate  parasitic  habit.  It  will  be  remem- 
bered, too,  that  the  hagfish,  in  which  hermaphroditism  is 
common,  is  also  to  a  large  extent  a  parasite.  But  what  Simon 
points  out  is,  that  organisms  on  which  great  demands  are  made, 
especially  in  the  way  of  muscular  exertion,  cannot  afford  to  be 
hermaphrodite ;  while  a  plethora  of  nutrition,  as  in  parasitism, 
tends  to  make  the  persistence  of  the  double  state  possible.  He 
gives  numerous  illustrations  of  this  very  reasonable  contention. 


7 8  THE    EVOLUTION    OF    SEX. 

Por  it  seems  plausible  that,  with  more  available  material  for 
internal  differentiation,  such  should  actually  occur.  But  it  is 
possible  to  venture  still  further. 

A  sluggish  habit  is  usually  associated  with  a  large  surplus  of 
nutritive  material,  and  at  the  same  time  very  frequently  with  an 
accumulation  of  waste  products.  Parasitism  means  not  only 
abundant,  but  rich  and  stimulating  nutrition.  Conditions 
which  combine  these  two  factors  w^ill  tend  to  secure  the  persist- 
ence of  primitive  hermaphroditism,  or  even  to  develop  it  from  a 
previously  attained  unisexual  state.  It  must  be  noted,  however, 
that  exceptions  occur,  which  it  is  at  present  difficult  to  explain. 
The  ctenophores  are  all  hermaphrodite,  yet  very  active.  So  too 
are  not  a  few  tunicates ;  while  the  brachiopods  are  extremely 
passive,  but  not  specially  characterised  by  hermaphroditism. 

§  lo.  Origin  of  Hermaphrodiiism. — There  can  be  very  little 
doubt  that  hermaphroditism  was  the  primitive  state  among 
multicellular  animals,  at  least  after  the  differentiation  of  sex- 
elements  had  been  accomplished.  In  alternating  rhythms,  eggs 
and  sperms  were  produced.  The  organism  was  alternately  male 
and  female.  Of  this  primitive  hermaphroditism,  there  is  probably 
more  or  less  of  a  recapitulation  in  the  life-history  of  all  animals. 
Gegenbaur  states  the  common  opinion  in  the  following  cautious 
and  terse  words : — "  The  hermaphrodite  stage  is  the  lower, 
and  the  condition  of  distinct  sexes  has  been  derived  from  it." 
Unisexual  "differentiation,  by  the  reduction  of  one  kind  of 
sexual  apparatus,  takes  place  at  very  different  stages  in  the 
development  of  the  organism,  and  often  when  the  sexual  organs 
have  attained  a  very  high  degree  of  differentiation."  The  first 
structural  stage  in  the  separation  would  probably  be  the  restric- 
tion of  areas,  in  which  the  formation  of  two  kinds  of  cells  still 
went  on  at  different  times  in  one  organism.  In  different  in- 
dividuals the  opposite  tendencies  we  have  already  spoken  of 
more  and  more  predominated,  till  unisexuality  evolved  out  of 
hermaphroditism. 

We  may  in  l:)rief  suggest  as  the  three  probal^le  grades  in  the  history  : — 
{a)  The  liberation  of  unindividuated  sex-elements  ;  (/»)  the  formation  of  two 
diverse  kinds  of  sex-elements,  incipiently  male  or  female,  at  the  same  time, 
or  at  different  periods,  according  to  nutritive  and  other  conditions  ;  (r)  the 
unisexual  outcome,  where  the  production  of  one  set  of  elements  has  pre- 
ponderated over  that  of  the  other. 

As  at  present  existing,  hermaphroditism  may  be  interpreted 

as  a  persistence  of  the  primitive  state,  or  as  a  reversion  to  it. 

Individual  cases  must  be  judged  by  themselves,  and  the  history 


HERMAPHRODITISM.  79 

of  each  must  be  taken  into  account.  But  where  the  hermaph- 
roditism is  manifestly  exceptional,  there  can  be  seldom  any 
question  in  regarding  it  as  a  reversion.  The  reversion  would 
generally  occur  on  the  female  side,  for  on  a  priori  physiological 
grounds,  it  is,  as  Simon  remarks,  more  readily  intelligible  that 
a  female  should  produce  sperms,  than  that  a  male  should 
produce  ova.  In  this  connection  it  is  interesting  to  notice 
how  Brock,  in  regard  to  the  development  of  the  reproduc- 
tive organs  of  snails,  maintains  that  they  are  laid  down  and 
developed  on  the  female  type,  and  only  become  secondarily 
hermaphrodite.  Purely  female  forms  still  occasionally  occur, 
which  he  interprets  as  exaggerations  of  the  side  normally 
preponderant.  So  in  hermaphrodite  bony  fishes,  the  same 
author  has  shown  that  the  preponderance  is  distinctly  female. 

Hermaphroditism  is  associated  in  some  cases  {eg.^  Polyzoa) 
with  the  occurrence  of  parthenogenesis  in  allied  forms ;  and  it 
may  be  noted,  as  will  become  clearer  afterwards,  that  for  a 
female  to  become  hermaphrodite  is  a  sort  of  step  towards 
parthenogenesis.  It  means  that  certain  cells  of  the  reproduc- 
tive organs  are  able  to  divide  of  themselves, — to  form,  however, 
not  an  embryo,  but  a  bundle  of  sperms. 

The  general  conclusion  then  is,  that  hermaphroditism  is  the 
primitive  condition,  and  that  the  cases  now  existing  either 
indicate  persistence  or  reversion. 


8o  THE    EVOLUTION    OF    SEX. 


SUMMARY 

1.  Hermaphroditism  is  the  union  of  the  two  sexual  functions  in  one 
organism.     This  occurs,  however,  in  varying  degrees. 

2.  Embryonic  hermaphroditism  is  probably  a  general  fact  with  even 
unisexual  animals.     It  is  certain  in  some  cases. 

3.  Casual  or  abnormal  hermaphroditism  is  not  infrequent. 

4.  Partial  hermaphroditism  (not  involving  the  essential  organs)  is 
exceedingly  common. 

5.  Normal  adult  hermaphroditism  ;  review  of  its  occurrence. 

6.  True  hermaphroditism  occurs  in  many  degrees  of  intimacy. 

7.  Self-fertilisation  is  a  rare  exception  among  animals ;  commoner  in 
plants. 

8.  "  Complemental  males" — pigmies  attached  to  hermaphrodites  — 
occur  in  two  groups. 

9.  The  conditions  of  hermaphroditism,  in  part,  involve  a  surplus  of 
material. 

10.  Hermaphroditism  is  primitive  ;  the  unisexual  state  is  a  subsequent 
d  fferentiation.  The  present  cases  of  normal  hermaphroditism  imply  either 
persistence  or  reversion. 


LITERATURE. 

See  already  cited  works  of 
Gegenbaur,  Hensen,  Hertwig,  Hatchett  Jackson  and  Rolleston, 

passim. 
Bourne. — On    Certain   Abnormalities   in    the    Common    Frog.       i.  The 

Occurrence  of  an  Ovotestis.     Quart.  J.  Micr.  Sci.,  XXIV. 
Brock. — Morph.  Jahrb.,  IV.     Beitriige  zur  Anatomic  und  Histologic  der 

Geschlechtsorgane  der  Knochenfische. 
Giles. — Quart.  Journ.  Micr.  Sci.      1888. 
Laulani£,  F. — Comptes  Rendus,  CI.  (1885),  pp.  393-5. 
Marshall,    A.     Milnes. — On   Certain    Abnormal    Conditions    of    the 

Reproductive  Organs  in  the  Frog.     Journ.   Anat.    Physiol.,  XVIII., 

pp.  121-44. 
Meehan,   T. — On  vSelf- Fertilisation  and   Cross-Fertilisation  in  Flowers. 

Penn.  Monthly,  VII.  (1876),  pp.  834-43. 
Pfluger,  E. — Archiv.  ges.  Physiol.,  XXIX. 
Simpson,  J.  Y. — Todd's  Cyclopredia  of  Anatomy  and    Physiology.     Art. 

Hermaphroditism,  pp.  684-738(1836-9). 
Stengel. — Arb.  Wiuzburg,  III.,  1876.     Ueber  d.  Urogenital  System  der 

Amphibien. 

Zwitterbildung  bei  Amphibien.      Biol,  Centrlbl.,  IV.,  8,  cf.  9. 

Sutton,  J.  B. — Hypertrophy  and  its  \'alue  in  Evolution.     Proc.   Zool. 

Soc,  London,  1888,  pp.  432. 
General  Pathology.     London,  1886, 


CHAPTER    VIL 

„.  •  'i^j 

The  Ultimate  Sex-Elements  {General  and  Historical). 

In  our  analysis  of  sex-characters  we  have  followed  the  general 
course  of  biological  history.  We  first  passed  from  the  form 
and  habit  of  a  male  or  female  organism  to  the  structure  and 
functions  of  the  sexual  organs.  In  discussing  hermaphroditism, 
we  had  occasion  to  refer  to  a  third  step  of  biological  analysis, 
that  which  involves  an  investigation  of  the  properties  of  the 


Mammalian  ovum,  showing  nucleolus  (ii),  nucleus  (/'),  yolk  (c), 
external  porous  zone  or  zona  pellucida  (^/),  and  follicular 
cells  ((')• — From  Hertwig,  after  Waldeyer. 

tissues.  Now  it  is  necessary  to  penetrate  deeper,  namely,  to  the 
sex-cells.  After  these  have  been  considered,  not  only  in  them- 
selves, but  finally  and  fundamentally  in  terms  of  the  changes  in 
the  p7-otoplasni  that  make  them  what  they  are,  then  we  shall  be 
in  a  better  position  to  re-ascend  to  some  of  the  problems  of 
reproduction.  ^  <^C*^ 


82  THE    EVOLUTION    OF    SEX. 

§  1.  TJie  Ovum  Theory. — Tt  is  now  a  commonplace  of 
observation  and  estal)lished  fact,  that  all  organisms,  reproduced 
in  the  ordinary  way,  start  in  life  as  single  cells.  We  see  insects 
laying  their  ova  upon  plants,  or  fishes  shedding  them  in  the 
water,  and  may  watch  how  these  cells,  provided  they  be 
fertilised,  give  rise  eventually  to  the  adult  organisms.  Con- 
veniently in  the  ordinary  frog-spawn  from  the  ditch,  we  can 
read,  what  was  for  so  long  a  riddle,  how  development  proceeds 
by  successive  cell-divisions  and  by  arrangement  of  the  multiple 
results.  Readily  seen  in  many  instances,  it  is  true  of  all  cases 
of  ordinary  sexual  reproduction,  that  the  organism  starts  from 
the  union  of  two  sex-cells,  or  that  it  is  with  the  division  of 
a  fertilised  ovum  that  development  begins. 

I'his  profound  fact,  technically  known  as  the  "ovum 
theory,"  has  been  not  unjustly  called  by  Agassiz  "  the  greatest 
discovery  in  the  natural  sciences  of  modern  times."  We  shall 
the  better  realise  the  magnitude  of  the  difference  which  its 
recognition  has  introduced  into  biology,  if  we  briefly  review 
the  history. 

§  2.  The  History  of  Einbryology^  Evolution  a?id  Epigenesis. 
—The  development  of  the  chick,  so  much  studied  in  embryo- 
logical  laboratories  to-day,  was  the  subject  of  inquiry  two  thou- 
sand years  ago  in  Greece.  Some  of  the  conspicuous  marvels  of 
reproduction  and  development  were  persistently  but  fruitlessly 
speculated  about  throughout  centuries.  It  was  only  during 
the  scientific  renaissance  of  the  seventeenth  century  that  the 
inquiry  became  more  keen  and  sanguine,  and  began  to  rely  to 
some  extent  at  least  on  genuine  observation. 

{a)  Harvey  (165 1),  with  the  aid  of  magnifying  glasses 
{perspecillce\  demonstrated  in  the  fowl's  egg  the  connection 
between  the  cicatricuta  of  the  yolk  and  the  rudiments  of 
the  chick,  and  also  observed  some  of  the  stages  of  uterine 
life  in  mammals.  More  important,  however,  were  his  far 
sighted  general  conclusions, — (i.)  That  every  animal  was  pro- 
duced from  an  ovum  {ovum  esse  primordium  commune  omniluis 
animalibus);  and  (2.)  That  the  organs  arose  by  new  formation 
{epigenesis),  not  from  the  mere  expansion  of  some  invisible  pre- 
formation. In  this  generalisation,  without  however  any  abandon- 
ment of  the  hypothesis  of  spontaneous  generation  of  germs,  he 
strove,  as  he  said,  to  follow  his  master  Aristotle,  and  was  in  so 
doing  as  far  ahead  of  his  contemporaries  as  a  strong  genius 
usually    is.       Before    Harvey,   the    observational    method    had 


JHE    ULTIMATE    SFX-ELEMENTS.  83 

indeed  begun.  Thus,  as  Allen  Thomson  notes,  Volcher  Goiter 
of  Groningen  (1573),  along  with  Aldrovandus  of  Bologna,  had 
Avatched  the  incubated  egg  in  its  marvellous  progress  from  day 
to  day.  Fabricius  ab  Aquapendente  (1621)  had  also  studied 
the  changes  in  the  incubated  egg,  and  the  stages  of  the 
mammalian  foetus.  In  keenness  of  vision,  Harvey  was  far 
ahead  of  either  of  these. 

(/»)  Malpighi  (1672),  using  a  microscope  with  phenomenal 
skill,  traced  the  embryo  back  into  the  recesses  of  the  cicatricula 
or  rudiment,  but  again  missed  a  magnificent  discovery,  and 
supposed  the  rudiments  to  have  pre-existed  in  the  egg.  In 
1677,  Leeuwenhock  was  led  by  Hamm  to  the  discovery  of 
the  spermatozoa ;  and  this  was  followed  up,  though  not  to 
much  profit,  by  Vallisneri  and  others.  Steno,  too,  in  1664, 
had  given  the  ovary  its  present  designation  ;  and  De  Graaf  had 
interpreted  the  vesicles  of  this  organ,  which  now  bear  his  name, 
as  for  the  most  part  equivalent  to  the  ova  which  he  had  dis- 
covered in  the  oviduct.  Needham  (1667),  Swammerdam  (1685), 
and  J.  van  Heme,  also  contributed  items  of  information  not 
then  appreciated  in  their  real  relations. 

(t)  The  Theory  of  Tirforniation —  Ovists  and  AnimaJculists. 
— In  the  early  part  of  the  eighteenth  century,  the  embryological 
observations  of  investigators,  like  Boerhaave,  were  summed  up 
in  the  conception  that  development  was  merely  an  expansion 
or  unfolding  of  a  pre-existent  or  preformed  rudiment  within  the 
egg.  Harvey  had  indeed  striven  for  an  opposite  conclusion, 
but  his  view  was  negatived,  as  we  have  seen,  by  Malpighi's 
failure  to  trace  the  embryo  beyond  the  rudiments  of  the 
cicatricula. 

The  notion  of  a  preformed  rudiment,  thus  suggested  by 
Boerhaave,  Malpighi,  and  others,  rapidly  became  the  prevalent 
theory.  In  so  far  as  it  emphasises  one  side  of  the  facts,  it  is 
bound  in  modified  form  so  to  remain.  Leibnitz,  Malebranche, 
and  others  found  it  to  fit  in  better  with  their  cosmic  concep- 
tions than  the  older  view  of  Aristotle  had  done,  and  welcomed 
it  accordingly. 

The  positions  occupied  by  the  physiologist  Haller  well 
illustrate  the  alterations  of  opinion.  As  Allen  Thomson  points 
out  in  his  article  "  Embryology,"  in  the  EncydopcEdia 
Britanuica^  "  Haller  was  originally  educated  as  a  believer  in 
the  doctrine  of  'preformation'  by  his  teacher  Boerhaave,  but 
was  soon  led  to  abandon  that  view  in  favour  of  '  e})igenesis '  or 


84  THE    EVOLUTION    OF    SEX. 

new  formation.  But  some  years  later,  and  after  having  been 
engaged  in  observing  the  phenomena  of  development  in  the 
incubaied  egg,  he  again  changed  his  views,  and  during  the 
remainder  of  his  life  was  a  keen  opponent  of  the  system  of 
epigenesis,  and  a  defender  and  exponent  of  the  theory  of 
'evolution,'  as  it  was  then  named." 

The  preformation  theory  found  more  and  more  definite 
expression  in  the  works  of  Bonnet,  Buffon,  and  others.  It  is 
now  necessary  to  sum  up  its  main  propositions. 

The  germ,  whether  egg-cell  or  seed,  was  believed  to  be  a 
miniature  model  of  the  adult.  "Preformed"  in  all  trans- 
parency lay  within  the  egg  the  future  organs,  only  requiring  to 
be  unfolded.  Bonnet  affirmed,  that  before  fertilisation  there 
lay  within  the  fowl's  ovum  an  excessively  minute  but  complete 
chick.  They  compared  the  germ  to  a  complex  bud,  which 
hides  within  its  hull  the  floral  organs  of  the  future.  Harvey 
had  said,  "the  first  concrement  of  the  future  body  grows, 
gradually  divides,  and  is  distinguished  into  parts ;  not  all  at 
once,  but  some  produced  after  the  others,  each  emerging  in  its 
order."  Very  different  was  Haller's  first  and  last  utterance, 
"  There  is  no  becoming ;  no  part  of  the  body  is  made  from 
another,  all  are  created  at  once."  This  was  obviously  a  short 
and  easy  method  with  embryology,  if  the  organism  was  literally 
preformed  in  the  germ,  and  its  development  simply  a  growth 
and  an  unfolding. 

But  this  was  not  all.  The  germ  was  more  than  a  marvellous 
bud-like  miniature  of  the  adult,  it  necessarily  included  in  its 
turn  the  next  generation,  and  this  the  next — in  short  all  future 
generations.  Germ  within  germ,  in  ever  smaller  miniature,  after 
the  fashion  of  an  infinite  juggler's  box,  was  the  corollary 
logically  appended  to  this  theory  of  preformation  and  unfold- 
ing,— of  evolution^  as  it  was  then  called,  in  a  very  different  but 
more  literal  sense  from  that  in  which  we  now  use  the  word. 

A  side  controversy  of  the  time  arose  between  two  schools, 
who  called  each  other  "ovists"  and  "animalculists."  The 
former  maintained  that  the  female  germ  element  was  the  more 
important,  and  only  required  to  be  as  it  were  awakened  by  the 
male  element  to  begin  the  process  of  unfolding.  The  animal- 
culists, on  the  other  hand,  asserted  the  claims  of  the  sperm  to  be 
the  bearer  of  the  miniature  nest  of  organism  within  organism, 
and  supposed  that  it  only  recjuired  to  be  fed  l)y  the  ovum  to 
enlarge  and  unfold  the  first  of  the  models  which  it  concealed. 


THE    ULTIMATE    SEX-ELEMENTS. 


85 


{d)  Wolff'' s  Reasse7-tion  of  Epigenesis. — The  above  ingenious 
construction  was  rudely  shaken  down,  however,  in  1759,  when 
Caspar  Friedrich  \V' olff  showed,  in  his  doctorial  dissertation,  the 
illegitimacy  of  the  suppositions  which  lay  at  the  root  of  the 


.    f  ~  , 


The  first  stages  of  development  in  a  number  of  animals.  A, 
Sponge,  Coral,  Earthworm,  or  Starfish  ;  B,  Crayfish  or 
other  Arthropod  ;  L\  Tunicate,  Lancelet,  &c. ;  /?,  Frog 
or  other  Amphibian. 

I.  Fertilised  ovum  ;  2.  Segmented  ovum,  a  ball  of  cells,  morula, 
or  blastosphere  ;  3.  'I'he  same,  after  further  division  or  in 
section  ;  4.   J  he  gastrula  stage. 

preformation  theory.  He  traced  the  chick  back  to  a  layer  of 
organised  particles  (the  familiar  cells  of  to-day),  in  which  there 
was   no  likeness   of  the  future  embryo,  far  less  adult.      More 


86  THE    EVOLUTION    OF    SEX. 

than  that,  he  followed  "  the  disposition  of  these  primitive 
elements  to  the  upbuilding  of  some  of  the  important  organs. 
He  undoubtedly  reached  too  far  in  his  emphasis  on  the  entire 
simplicity  of  the  germ,  and  many  of  his  details  were  mistaken ; 
but  none  the  less  did  he  recall  embryologists  from  speculation 
to  take  the  facts  as  they  found  them,  and  lay  the  foundation  of 
modern  embryology  in  the  fact  that  organisation  was  gradually 
acquired  by  an  observable  process  of  development. 

(<?)  Wolff's  Successors.— ^\v^  important  conclusion  reached 
by  Wolff  remained  for  about  sixty  years  without  effect.  In  i8i  7, 
Christian  Pander  took  up  embryological  research  exactly  where 
Wolff  had  left  it,  and  worked  out  the  history  of  the  chick  in 
more  exact  detail.  In  1824,  Prevost  and  Dumas  noticed  the 
division  of  the  ovum  into  masses ;  and  in  the  following  year 
Purkinje  discovered  the  nucleus  or  "germinal  vesicle."  Von 
Baer  followed  up  his  friend  Pander's  work,  and  in  1827  made 
the  memorable  discovery  of  the  mammalian  ovum,  which  he 
traced  from  uterus  to  oviduct,  and  then  to  its  position  in  the 
ovary  itself.  Thus,  after  a  century  and  a  half,  De  Graafs 
endeavour  was  at  length  fulfilled.  Soon  afterwards,  Wagner, 
von  Siebold,  and  others,  elucidated  what  was  still  hidden  from 
von  Baer, — the  real  nature  of  the  spermatozoa.  Meanwhile, 
Bichat's  analysis  (1801)  of  the  organism  into  tissues,  was  with 
improved  appliances  deepened  in  the  casual  description  of 
"cells";  and  an  important  generalisation  had  its  forecast  in 
1835,  when  Johannes  Miiller  pointed  out  in  the  vertebrate 
notochord  the  existence  of  cells  resembling  those  of  plants. 

>5  3.  The  Cell-Theory.  —  Without  continuing  the  history 
further,  we  must  simply  note  that  in  1838  Schleiden  referred 
all  vegetable  tissues  to  the  cellular  type,  and  traced  back  the 
plant  embryo  to  a  single  nucleated  cell ;  while,  in  the  following 
year,  Schwann  boldly  extended  this  conception  of  plant  struc- 
ture and  development  to  the  animal  world,  and  so  fully  consti- 
tuted the  "cell-theory."  The  ovum,  recognised  as  a  cell, 
became  a  "  primordium  commune  "  in  a  deeper  sense  than 
Harvey  dreamt  of;  the  masses  described  by  Prevost  and  Dumas 
were  seen  as  the  products  of  cell  division;  and  Kolliker  led  the 
way,  now  so  well  followed  up,  in  tracing  these  cells  to  their 
results  in  the  tissues  of  the  organism. 

§  4.  Protoplasmic  Basis. — Only  one  step  further  is  it  possible 
for  biological  analysis  to  penetrate,  and  that  within  the  last  few 
years  is  being  persistently  essayed.      It  is  impossible  to  rest  at 


THE    ULTIMATE    SEX-ELEMENTS. 


87 


the  cell-theory  level.  To  recognise  the  ovum  as  a  cell,  and  the 
spermatozoon  as  another,  to  find  the  starting-point  of  the 
organism  in  the  double  unity  formed  from  these  two,  to  demon- 
strate the  process  of  development  as  one  of  cell  multiplication 
and  arrangement,  express  great  but  not  final  biological  facts. 
Thus  it  is  that  of  late  years,  what  Michael  Foster  has  called  the 
"protoplasmic  movement"  has  made  itself  felt,  not  only  in 
study  of  the  general  functions  of  the  body,  but  in  the  special 
physiology  of  the  reproductive  cells  and  their  history.  Even 
in  morphological  or  structural  studies,  attention  has  shifted 
from  the  shapes  of  cells  to  the  structure  of  their  living  matter, 
or  from  the  different  forms  of  ovum  and  spermatozoon  to  the 
germinal  protoplasm  or  Keimplasma  which  they  contain.     On 


Ground  Plan  of  Protoplasmic  Changes. 

this  level,  in  fact  wliere  biology  has  touched  the  bottom, 
morphology  and  physiology  have  become  more  than  ever 
inseparable.  All  the  facts  of  structure  on  the  one  hand,  and  of 
function  on  the  other,  have  both  to  be  interpreted  in  terms  of 
the  constructive  and  disruptive  changes  in  the  living  matter 
itself.  The  general  theory  may  be  summarised  in  the  accom- 
panying diagram.  Protoplasm  is  regarded  as  an  exceedingly 
complex  and  unstable  compound,  undergoing  continual  mole- 


88 


THE    EVOLUTION    OF    SEX. 


cular  change  or  metabolism.  On  the  one  hand,  more  or  less 
simple  dead  matter  or  food  passes  into  life  by  a  series  of 
assimilative  ascending  changes,  with  each  of  which  it  becomes 
molecularly  more  complex  and  unstable.  On  the  other  hand, 
the  resulting  protoplasm  is  continually  breaking  down  into  more 
and  more  simple  compounds,  and  finally  into  waste  products. 
The  ascending,  synthetic,  constructive  series  of  changes  are 
termed  "anal)olic;"  and  the  descending,  disruptic  series,  "kata- 
bolic."  Both  processes  may  be  manifold,  and  the  predominance 
of  a  particular  series  of  anabolic  or  katabolic  changes  implies 
the  specialisation  of  the  cell.  The  upper  figure  (a)  represents 
the  complex  unstable  protoplasm  as  if  occupying  the  summit  of 
a  double  flight  of  steps ;  it  is  formed  up  the  anabolic  steps,  it 
breaks  up  and  descends  by  the  katabolic.  The  lower  figure  (b) 
is  a  projection  of  the  other,  its  convergent  and  divergent  lines 
serving  to  represent  the  various  special  lines  of  anabolism  and 
katabolism  respectively,  and  the  definite  component  substances 
("anastates"  and  "  katastates ")  which  it  is  the  task  of  the 
chemical  physiologist  to  isolate  and  interpret  (see  pp.  122-4). 


1  Vi_;X      ^^  .^jHPi,        ^-^-^-^      j^^St^s.  V:^     f-l 


,^^W^J&^ 


^  m::^ 


^  ©®®© 


Protospongi.1,  a  rolonial  infusorian,  showiiig  the  difference  between 
outer  antl  inner  cells. — From  Saviile  Kent. 

§  5.  Protozoa  and  Mctazoa. — It  has  been  emphasised  above 
that  every  multicellular  organism,  reproduced  in  the  ordinary 
way,  starts  from  a  fertilised  ovum,  from  what  may  be  fairly 
called  a  single  cell.     Sponge,  butterfly,  bird,  and  whale  start  at 


THE    ULTIMATE    SEX-ELEMENTS. 


89 


the  level  of  the  simplest  animals  or  Protozoa,  which  (with  the 
exception  of  very  loose  colonies)  remain  always  unicellular. 
The  simplest  organisms  leave  off  where  the  higher  plants  and 
animals  begin,  i.e.,  as  unit  masses  of  living  matter.  They  corre- 
spond, in  fact,  to  the  reproductive  cells  of  higher  animals,  and 
may  be  called,  according  to  their  predominant  character,  protova 
and  protosperms.  A  fertilised  ovum,  as  we  have  seen,  pro- 
ceeds by  division  to  form  a  "body;"  the  Protozoon  remains, 
with  few  exceptions,  a  single  cell,  in  which  there  is  obviously 
no  distinction  between  reproductive  elements  and  entire 
organism. 

Reference  will  have  to  be  made  to  the  Protozoa  in  three 
connections,  which  may  be  here  simply  noted  : — 

{a)  In  their  chief  groups,  and  in  the  stages  of  their  life- 


Ophrydium,  a  colonial  infusorian.— From  Sa\  ille  Kent. 

histories,  they  express  phases  in  the  same  cell  cycle  which  recurs 
in  higher  forms  in  the  component  elements  of  the  body,  and 
in  the  reproductive  cells.  The  contrast,  in  other  words,  between 
an  infusorian  and  an  amoeba,  between  the  ciliated  and  amceboid 
stage  in  the  life-history  of  many  forms,  is  a  forecast  of  the 
contrast  between  a  ciliated  cell  and  a  white  blood  corpuscle, 
between  a  mobile  spermatozoon  and  a  young  ovum.  That  is 
to  say,  a  predominance  of  the  same  protoplasmic  processes  is  the 
common  explanation  of  such  similarities  of  form  (see  p.  121). 

{b)  It  is  among  the  Protozoa  that  we  must  presently  look,  if 
we  hope  to  understand  the  origin  and  import  either  of  "male 
and  female,"  or  of  fertilisation  (see  pp.  119,  128). 

{c)  Among  the  loose  colonies  which  some  Protozoa  form. 


90  THE    EVOLUTION    OF    SEX. 

and  which  bridge  the  gulf  between  the  unicellular  animals  and 
the  Metazoa,  there  is  seen  the  beginning  not  only  of  the  formation 
of  a  " body, "  but  also  the  setting  apart  of  special  reproductive  cells 
(see  figs,  oh  pp.  88,  89).  On  this  point  more  emphasis  must  be 
laid.  The  ordinary  Protozoon  is  a  single  cell,  and  forms  no  body. 
It  divides  indeed,  and  multiplies  accordingly,  but  the  products 
of  division  go  asunder,  whereas  in  the  segmentation  of  the  ovum 
they  remain  connected.  In  most  Protozoa,  there  is  continual 
self-recuperation  ;  in  most,  division  occurs  without  any  loss  ;  in 
most,  there  is  no  distinction  between  parent  and  offspring ;  in 
most,  as  there  is  no  body,  there  is  no  death.  Thus  it  is  that, 
with  one  weighty  caution  to  be  afterwards  noted,  it  seems  justi- 
fiable to  speak  with  Weismann  and  others  of  the  "immortality  of 
the  Protozoa."  In  a  certain  sense  too,  as  we  shall  see,  it  is 
justifiable  to  speak  of  the  immortality  of  the  reproductive  cells 
in  higher  animals.  The  body  dies,  but  the  reproductive  cells 
escape,  before  its  death,  to  live  on,  as  new  organisms,  enclosing 
new  sets  of  reproductive  cells.  Again  there  is  similarity  between 
the  Protozoa  and  the  reproductive  cells. 

But  in  some  of  the  loose  colonies  {e.g.,  Vo/vox),  we  see  the 
beginning  of  the  change  which  introduced  death  as  a  constant 
phenomenon  (see  fig.  p.  130).  The  cell,  which  starts  one  of 
these  colonies,  divides;  the  products  of  division,  instead  of  going 
apart  as  usual,  remain  connected  ;  a  loose  body  of  many  cells 
is  thus  formed.  In  this  cluster  of  cells,  certain  elements  are  in 
turn  set  apart  and  eventually  adrift,  as  reproductive  cells.  They 
start  new  colonies,  and  thus  we  are  introduced  to  what  is  con- 
stant in  higher  animals.  The  only  marked  differences  are — (a) 
that  the  body  of  the  Metazoon  is  more  than  a  loose  colony  of 
cells ;  (^)  that  the  reproductive  elements  are  usually  liberated 
from  some  definite  region  or  organ ;  and  (c)  that  they  are  more 
markedly  differentiated  as  male  and  female  cells. 

§  6.  Genei'iil  Origiti  of  iJie  Sex-Cells. — Except  in  the  lowest 
invertebrates,  the  sponges  and  coelenterates,  the  reproductive 
elements  almost  always  arise  in  connection  with  the  middle 
layer  (mesoderm  or  mesoblast)  of  the  body. 

Neither  in  sponges  nor  in  cnelenterales  is  there  a  middle  layer  exactly 
comparahle  to  the  mesoderm  of  higher  animals  ;  the  less  definite  middle 
stratum  is  now  frequently  termed  a  mesogkea.  In  sponges,  we  already 
mentioned  that  the  reproductive  cells  simply  arise  here  and  there  among 
the  other  elements  of  the  stratum.  The  ova  are  highly  nourished  mesoglceal 
cells  ;  the  piimilive  male  cells,  which  divide  into  numerous  minute  sperma- 
tozoa, are  the  reverse. 


THE    ULTIMATE    SEX-ELEMENTS.  9 1 

In  ccelenterates  the  phenomena  are  of  much  interest  ;  the  origin  of  the 
sex-cells  is  very  diverse.  Some  time  ago  considerable  emphasis  was  laid, 
by  E.  van  Beneden  and  others,  on  the  fact  that,  in  certain  Hydrozoa,  "  the 
ova  are  derived  from  the  endoderm,  and  the  sperms  from  the  ectoderm." 
Thus  Gegenbaur,  accepting  this,  remarks  that  in  such  cases  "  the 
endoderm  is  the  female,  and  the  ectoderm  the  male  germinal  layer." 
Such  a  generalisation,  if  estal)lished,  would  be  plausible  enough,  seeing 
that  the  inner  or  endoderm  layer  is  the  more  nutritive  or  anabolic  of  the 
two.  A  controversy  however  soon  arose,  the  result  of  which  was  to  over- 
throw the  generalisation.  In  hydra,  we  have  already  noticed  that  both 
products  arise  from  the  ectoderm  ;  the  same  was  shown  by  Ciamician  to  be 
true  of  Tubitlaria  mescnibryanthcmiiin  ;  while  in  the  Eiidcndriit)n  ramosiivi 
the  ova  appeared  to  arise  from  the  ectoderm^  and  the  male  elements  from 
the  endoderm,  the  very  reverse  of  Van  Beneden's  conclusion.  The  matter 
was  settled,  so  far  as  the  general  facts  are  concerned,  by  Weismann,  who 
established  the  fact  of  active  migration  of  the  elements  from  one  layer  to 
another.  He  has  since  been  followed  l»y  other  investigators,  [a)  The  sex- 
elements,  both  male  and  female,  may  appear  first  in  the  endoderm,  whether 
they  originate  there  or  not,  and  from  this  inner  layer  they  migrate  to  the 
ectoderm,  where  they  ripen.  [li)  In  rare  cases  ihey  even  ripen  in  the 
endoderm.  {c)  Very  commonly  the  sex-cells  originate  in  the  ectoderm  and 
ripen  there,  or  they  may  pass  thence  into  the  endoderm  and  back  again  to 
the  ectoderm,  [d)  In  the  medusa  of  Obelia,  the  ova  appear  to  ripen  partly 
in  both  layers.  These  facts,  a  convenient  summary  of  which  will  be 
found  in  Hatchett  Jackson's  erudite  edition  of  Rolleston's  "  Forms  of 
Animal  Life,"  show  plainly  enough  how  varied  are  the  origin  and  history 
of  the  sex -cells  in  these  forms. 

The  colonial  hydroids  typically  produce  well-marked  reproductive 
individuals  or  sexual  zooids,  set  free  as  "  swimming-bells  "  or  medusoids 
(in  a  process  to  be  afterwards  described  under  "  Alternation  of  (jenera- 
tions  ").  In  these  the  reproductive  elements  are  typically  developed.  But 
in  varying  degrees  these  medusoids  have  degenerated,  and  are  frequently 
not  only  not  liberated,  but  lose  their  characteristic  features,  and  become 
mere  reproductive  buds.  In  these  buds  the  sex-cells  are  normally 
developed.  But  it  very  frequently  happens  that  they  arise  more  or  less  in 
the  body  of  the  asexual  vegetative  hydroid.  They  ripen  early,  and  sub- 
sequently migrate  to  their  proper  place  ;  the  asexual  stage  incorporating 
more  and  more  of  the  originally  separate  sexual  generation.  Weismann  has 
emphasised  the  value  of  this  early  ripening  as  an  advantage  to  the  race, 
lessening  the  danger  of  its  extinction;  and  this  has  doubtless  to  be  con- 
sidered, though  it  can  hardly  be  regarded  as  a  physiology  of  the  facts. 

§  7.  Early  Separation  of  Sex- Cells. — Having  noted  the 
general  fact  of  mesodermic  origin,  and  some  of  the  interesting 
phenomena  observed  in  ccelenterates,  we  shall  not  further  pursue 
the  subject  except  as  regards  one  question,  the  period  at  which 
the  reproductive  cells  make  their  appearance.  This  is  sometimes 
early,  sometimes  late  ;  and  it  is  not  yet  decisively  known  how 
widely  early  separation  occurs,  nor  how  far  the  fact  is  of  much  sig- 
nificance. The  question  will  have  to  be  discussed  in  the  volume 
treating  of  heredity  ;  only  a  brief  reference  is  here  possible. 


92  THE    EVOLUTION    OF    SEX. 

In  the  case  of  a  well-known  fly,  Chironoffiiis,  Prof.  Balbiani, 
unprejudiced  by  any  theory  of  heredity,  observed  the  following 
facts  : — Before  the  segmentation  of  the  egg  had  at  all  advanced, 
before  what  embryologists  call  the  blastoderm  was  more  than 
incipient,  two  cells  were  observed  to  be  set  apart  externally. 
(These  had  nothing  whatever  to  do  with  the  polar  globules 
seen  in  most  ova  at  maturation.)  The  development  proceeded 
apace,  but  the  isolated  cells  took  no  share ;  they  may  be  pre- 
sumed to  have  retained  intact  the  characters  which  they 
received  when  first  divided  off  from  the  ovum.  At  a  certain 
stage,  however,  the  insulated  cells  sank  inwards,  took  up  an 
internal  position,  became  the  rudiments  of  the  reproductive 
organs.  Here  then,  at  an  early  stage,  before  diff"erentiation  is 
marked,  the  reproductive  cells  are  set  apart.  They  must 
therefore  preserve  much  of  the  character  of  the  parent  ovum, 
and  hand  on  the  tradition  intact  by  continuous  cell-division  to 
the  next  generation. 

In  other  words,  in  the  preceding  case,  at  a  very  early  stage 
in  the  embryo,  the  future  reproductive  cells  are  distinguishable 
and  separable  from  the  body-forming  cells.  'J'he  latter  develop 
in  manifold  variety,  into  skin  and  nerve,  muscle  and  blood,  gut 
and  gland ;  they  differentiate,  and  lose  almost  all  protoplasmic 
likeness  to  the  mother  ovum.  But  the  reproductive  cells  are 
set  apart ;  they  take  no  share  in  the  differentiation,  but  remain 
virtually  unchanged,  and  continue  unaltered  the  protoplasmic 
tradition  of  the  original  ovum.  After  a  while  they,  or  their 
division-products  rather,  will  be  liberated  as  reproductive  cells. 
These  in  a  sense  will  be  continuous  with  the  parental  germ. 
Their  ])rotoplasm  will  be  more  or  less  identical.  The  original 
ovum  has  certain  characteristics,  a  b  c ;  it  divides,  and  all  its 
cells  must  at  first  more  or  less  share  these  characteristics ;  the 
body-cells  lose  them,  the  insulated  reproductive  cells  must 
retain  them.  The  ovum  of  the  next  generation  has  thus  also 
the  characteristics  a  b  r,  and  must  therefore  produce  an 
organism  essentially  like  the  parent. 

An  early  isolation  of  the  reproductive  cells,  though  never 
so  striking  as  in  CJiirononius^  has  been  observed  in  many 
cases, — e.g.,  in  other  insects,  in  the  aberrant  worm-type 
Sa(:;itta.,  in  leeches,  in  thread-worms  or  nematodes,  in  some 
Polyzoa,  in  some  small  crustaceans  known  as  Clndocera,  in  the 
water-flea  Moifia,:iU(\  in  some  spiders  {P/ui/a?igid(e).,^\-\d  probably 
in  other  cases.     As  the  series  is  ascended,  the   reproductive 


THE    ULTIMATE    SEX-ELEMENTS.  93 

organs  are  later  in  making  their  appearance,  or  at  least  they  are 
only  detected  at  a  later  stage ;  and  it  must  also  be  pointed  out 
that,  in  cases  of  alternation  of  generations,  an  entire  asexual 
generation,  or  more  than  one,  may  intervene  between  one  ovum 
and  another. 

§  8.  Body  Cells  and  Reproductive  Cells. — Various  naturalists 
have  insisted  on  the  contrast  hinted  at  above,  between  the  cells 
of  the  embryo  which  go  to  form  the  body,  and  those  which  are 
set  apart  as  reproductive  organs. 

(<?)  As  early  as  1849,  Owen  noted  that,  in  the  developing 
germ,  it  was  possible  to  distinguish  between  cells  which  became 
much  changed  to  form  the  body,  and  cells  which  remained 
little  changed  and  formed  the  reproductive  organs.  This  view, 
as  Brooks  points  out,  he  unfortunately  afterwards  departed  from 
in  his  Anatomy  of  the  Vertebrates. 

{b)  In  1866,  Hoeckel  connected  reproduction  with  discon- 
tinuous growth,  and  insisted  upon  the  material  continuity 
between  parent  and  offspring.  Somewhat  later,  both  he  and 
Rauber  drew  a  clear  contrast  between  the  somatic  and  repro- 
ductive elements,  between  the  "  personal  "  and  "  germinal  " 
portions  of  the  embryo,  or  between  the  l)ody  and  the  sex  cells. 

{c)  W.  K.  Brooks,  in  1876  and  1877,  again  drew  attention 
to  this  significant  contrast. 

{d)  Yet  more  explicit,  in  1877,  was  the  ingenious  Dr 
Jager,  now  better  known  in  a  very  different  connection,  and 
a  few  of  his  sentences  well  deserve  to  be  quoted.  Referring 
to  a  previous  paper,  he  writes  as  follows : — "  Through  a 
great  series  of  generations,  the  germinal  protoplasm  retains  its 
specific  properties,  dividing  in  every  reproduction  into  an 
ontogenetic  portion,  out  of  which  the  individual  is  built  up, 
and  a  phylogenetic  portion,  which  is  reserved  to  form  the  re- 
productive material  of  the  mature  offspring.  This  reservation 
of  the  phylogenetic  material  I  described  as  t/ie  continuity  of  the 
gcrni-p7'otoplasni.  Encapsuled  in  the  ontogenetic  material,  the 
phylogenetic  protoplasm  is  sheltered  from  external  influences, 
and  retains  its  specific  and  embryonic  characters." 

ie)  In  an  exceedingly  clear  manner,  to  which  sufficient 
attention  seems  hardly  to  have  been  accorded,  Galton,  in 
1876  and  at  other  dates,  as  again  more  indirectly  in  his  recent 
A^atural  Inheritance.,  drew  attention  to  the  contrast  between  the 
gemmules  of  the  ovum  (stirp)  which  go  to  form  the  body,  and 
those  which,  remaining  undeveloped,  form  the  sex-cells.     "  The 


94 


THE    EVOLUTION    OP^    SEX. 


■'({ 


A 


developed  part  of  the  stirp  is  almost  sterile  "  (i.e.,  without  in- 
fluence in  heredity)  ;  "  it  is  from  the  unde- 
velo{)ed  residue  that  the  sexual  elements  are 
derived." 

(/)  Lastly,  in  1880,  Nussbaum,  in  an 
elaborate  investigation  on  the  differentiation 
of  the  reproductive  cells,  drew  emphatic 
attention  to  some  cases  of  their  early  separa- 
tion, and  reasserted  Jager's  conception  of 
a  continuity  of  germ-])rotoplasm.  In  this 
survey,  however,  we  do  not  pretend  to  decide 
the  difficult  question  of  priority  in  the 
enunciation  of  this  conception.  Like  many 
other  generalisations,  it  appears  to  have 
arisen  all  but  simultaneously  in  many  minds. 
>^  9.  IVeismann^s  Theory  of  the  Continuity 
of  the  Germ  -  Protoplasm. — In  some  cases 
referred  to  in  a  foregoing  paragraph,  it  is 
possible  to  trace  a  direct  cellular  continuity, 
first  of  all,  between  the  ovum  and  early 
separated  reproductive  rudiments ;  secondly, 
between  the  latter  and  the  future  ova  and 
sperms.  There  is  not  only  cellular  continuity 
l)etween  the  ovum  which  gives  rise  to  parent, 
and  the  ovum  which  gives  rise  to  offspring, 
— that  the  cell-theory  demands, — but  there  is 
a  continuity  in  which  the  character  of  the 
original  ovum  is  never  lost  by  differentia- 
tion. In  fact,  there  is  a  continuous  chain 
of  reproductive  cells  quite  apart  from  the 
body  cells.  It  is  in  this  sense  that  some  of 
the  authors  quoted  have  si)oken  of  the  con- 
tinuity of  the  germ-ri7A\  This  is  certainly 
true  for  some  cases.     If  it  were  true  for  all, 

cession    of   Protozoa;  |-}-,^   problems   of  rcproductiou  and   heredity 

lurttier  on,  It  represents  111  1         •  i  1 

the  ova  from  which  the  would  bc  much  smiplcr  than  they  at  present 

"bodies"       (undotted)  ^^^r^pT,-  f^  l.p 
are  produced.     At  each  •^q^P^''^^  ^O   Oe. 

generation,  a  sperma-         For    in    the   prcscnt   statc   of   our  know- 

tozoon     fertilisintr     the  1      i  i  \  r    .\  .-• . 

liberated  ovum  is  also  l^dgc   wc   can   Only  spcak  ot   tlic  Continuity 

indicated.  Qf   ^^^    rcproductivc    cclls.,  \\\  cxccptiona" 

rather  in  a  small  minority  of  cases.     Alike  in  the 

brates    and    the    lowly    hydroids,   the    reproductive    cells   may 


The  relation  between  re- 
productive cells  and  the 
body.  The  continuous 
chain  of  dotted  cells  at 
first    represents    a    suc- 


or 
higher  verte- 


THE    ULTIMATE    SEX-ET.EMENTS.  95 

appear  late.  After  the  differentiation  of  the  vertebrate  embryo 
has  progressed  far,  or  the  Hfe  of  the  polyps  continued  for  long, 
the  germ-cells  make  their  appearance  ;  and  though  we  know 
of  course  that  they  are  descendants  of  the  original  ovum,  yet 
we  must  allow,  with  Weismann,  that  in  the  form  of  special  cells 
they  are  now  for  the  first  time  to  be  detected.  Therefore, 
Weismann  says,  "a  continuity  of  Qerm-re/Zs  is  now  for  the  most 
part  no  longer  demonstrable." 

Yet  there  is  nothing  that  Weismann  more  strongly  insists 
upon,  than  the  reality  of  continuity  between  ovum  and  ovum. 
In  what  does  it  consist,  if  a  chain  of  ovum-like  cells  is  only 
true  of  a  minority  of  organisms  ?  It  consists,  according  to 
Weismann,  in  the  "  Keimplasma  "  or  germ-protoplasm. 

The  germ-plasma  is  the  distinctive  part  of  the  nucleus  of  the 
germ-cell.  It  has  an  extremely  complex,  and  at  the  same  time 
persistent,  structure.  It  is  the  substance  which  enables  the 
germ-cell  to  build  up  an  organism,  the  architectural  living 
matter,  and  the  immortal  bearer  of  all  properties  transmitted  in 
inheritance.  "  In  every  development,"  according  to  Weismann, 
"a  portion  of  this  specific  germ-i)lasma,  which  the  parental 
ovum  contains,  is  unused  in  the  upbuilding  of  the  offspring's 
body,  and  is  reserved  unchanged  to  form  the  germ-cells  of  the 
next  generation.  .  .  .  The  germ-cells  no  longer  appear  as 
products  of  the  body,  at  least  not  in  their  most  essential  part — the 
specific  germ-plasma  ;  they  appear  rather  as  something  opposed 
to  the  sum-total  of  body-cells  ;  and  the  germ-cells  of  successive 
generations  are  related  to  one  another  like  generations  of  Pro- 
tozoa." But  the  continuity  is  rarely  kei)t  up  by  a  chain  of 
undifferentiated  reproductive  ce//s ;  it  depends  upon  the  con- 
tinuance and  unchanged  persistence  of  a  minimal  quantity  of 
the  original  germ-plasma. 


96  THE    EVOLUTION    OF    SEX. 


SUMMARY. 

The  progressive  analysis  through  organism,  organs,  tissues,  and  cells,  to 
the  living  matter  itself. 

1.  The  Ovum-theory. — Every  organism,  reproduced  in  the  ordinary 
way,  arises  from  a  fertilised  egg-cell,  and  development  proceeds  by  cell- 
division. 

2.  Epigenesis  and  Evolution. — Plistory  of  the  different  views  taken  of 
the  development  of  the  organism  ;  ancient  speculations.  The  scientific 
renaissance,  {a)  Harvey's  prevision  of  the  ovum-theory,  and  emphasis  upon 
"epigenesis."  {/>)  Observations  of  Malpighi  and  others,  mostly  against 
Harvey's  view,  {c)  The  theory  of  preformation, — of  a  nest  of  miniature 
models  within  the  egg,  only  requiring  to  be  unfolded  in  successive  genera- 
tions ;  Ovists  z'^ri'/^J'Animalculists.  (</)  VVolfiCsreassertion  of  "epigenesis," 
the  foundation  of  modern  embryology  ;  his  exaggeration  of  the  simplicity 
of  the  germ.     (1?)  Wolff's  successors. 

3.  The  Cell-Theory.  — All  organisms  are  made  up  of  cells,  and  start  from 
cells. 

4.  A  protoplasmic  basis  now  being  laid.  The  "germ-plasma"  more 
important  than  the  egg-cell.  All  to  be  explained  in  terms  of  protoplasmic 
changes. 

5.  The  contrast  between  Protozoa  and  Metazoa. — The  making  of  a 
"  body  "  as  distinct  from  reproductive  cells. 

6.  General  origin  of  the  sex-cells,  indefinite  in  sponges,  variable  in 
cuelenterates,  generally  from  the  mesoderm  in  higher  animals. 

7.  Early  separation  of  the  reproductive  cells  to  be  seen  in  a  minority  of 
cases. 

8.  The  contrast  between  somatic  and  reproductive  cells,  and  the  con- 
tinuity of  the  latter  ;  Owen,  Hreckel,  Rauber,  Brooks,  Jager,  Gallon, 
Nussbaum. 

9.  Weismann's  theory  of  the  continuity  of  the  ger\w-/>/as///a  (a  specific 
nuclear  matter),  as  opposed  to  continuity  by  a  chain  of  undiflerentiated 
cells,  which  is  known  to  occur  only  in  a  minority  of  organisms. 


LITERATURE. 

For    relevant  literature    and    further    details,    consult    the  Text-books   of 

Balfour,  lladdon,  and  Ilertwig  ;  also, 
Geddks,    p. — EncyclopiTcdia  Britannica  articles  already  referred   to;  also 

MORPIIOI.OC.Y,  ibid. 
IlrcNSKN,  V.  —  Op.  fit. 

M'Kenm)i;ici<,  J.  G. — Text-book  of  Physiology.      Lond.,  1888. 
Thomson,    J.    A. — Arts.     Cell    and    Embryology,    new  Edition   of 

Chambers's  Encyclopredia. 

History  and  Theory  of  Heredity.      Proc.  Roy.  Soc.  Edin..  1888. 

Wai.deykr,  W. — Die  Karyokinese,  &c.     Arch.  Mikr.  Anat.,  1888. 

Wkismann.  —  0pp.  cit. 

Zoological  Record,  General  Subjects:  Cell,  Oogenesis,  &c.,  since  1886. 


CHAPTER   VIII. 

The  Egg-Cell  or  Ovurl 

In  the  preceding  chapter  we  sketched  the  history  of  the  "  ovum- 
theory,"  which  expresses  the  now  famihar  fact  that  every 
organism,  reproduced  in  the  ordinary  way,  develops  from  a 
fertiHsed  egg-cell.  It  is  now  necessary  to  attend  more  carefully 
to  the  essential  characters  and  history  of  this  "  primordium 
commune,"  this  common  starting-point  of  life,  leaving  the  details. 


Animal  Cell,  showing  the  chromatin  elements  of  nucleus 
{a)  in  a  long  coil,  and  the  protoplasmic  network  (d)  round 
about. — From  Carnoy. 

along   with  the  other  problems  of  development,   to  a  special 
volume  devoted  to  Embryology. 


§  I.  Structure  of  the  Ovum. 


essential    features    of  any 


other 

G 


-The   ovum  presents  all    the 
animal   cell.     There    is    the 


98 


THE    EVOLUTION    OF    SEX. 


cell-substance,  consisting  in  part  of  genuine  living  matter  or 
protoplasm  ;  and  there  is  the  nucleus,  or  "  germinal  vesicle," 
which  plays  such  an  important  part  in  the  ripening,  fertilising, 
and  subsequent  division  of  the  cell. 

The  cell-substance  exhibits,  when  highly  magnified,  a  homo- 
geneous matrix,  traversed  by  a  delicate  network,  with  minute 
yolk-balls,  pigment,  and  other  granules  strewn  about  the  meshes. 
So  much  of  it  is  genuine  protoplasm,  of  course,  but  then  there 
are  also  substances  in  process  of  ascent  and  even  descent  from 
the  climax  of  living  matter,  and  there  is  in  more  or  less  abund- 


Ovum  of  a  Threadworm  (Ascar/s),  showing  (a)  the 
chromatin  elements  of  the  nucleus,  and  the  appear- 
ance of  the  surrounding  yolk. — From  Carnoy. 

ance  a  reserve  capital  of  yolk  nutriment  for  the  future  embryo. 
Delicate  observations,  by  the  modern  masters  of  microscopic 
technique,  have  detected  many  marvels  in  the  egg-cell,  into 
which  which  we  cannot  at  present  enter.  Thus,  within  the  last 
year,  Boveri  has  drawn  attention  to  a  special  element  in  the  pro- 
toplasm, which  he  calls  arc/io/>/asm,  a  substance  which,  as  its 
name  suggests,  seems  to  have  an  altogether  marvellous  architec- 
tural function  in  relation  to  the  changes  of  the  nucleus  in 
segmentation. 


THE    EGG-CELL    OR    OVUM.  99 

When  Purkinje,  in  1825,  discovered  the  nucleus  of  the 
fowl's  egg,  he  could  have  little  idea  that  the  little  "vesicle"  to 
which  he  directed  the  attention  of  investigators  was  in  reality  an 
intricate  microcosm.  I  >ittle  more  than  ten  years  elapsed,  before 
R.  Wagner  began  to  complicate  matters  by  the  discovery  of  the 
nucleolus  or  germinal  "spot"  within  the  "vesicle."  We  now 
know  that  the  nucleus  has  not  only  a  very  complex  structure, 
but  in  a  sense  a  curious  internal  life  all  its  own. 

The  nucleus,  when  quiescent,  often  lies  in  a  little  nest  or 
chamber  within  the  cell-substance,  and  is  limited  from  the  latter 
by  a  more  or  less  distinct  nuclear  membrane,  which  disappears 
as  the  period  of  activity  begins.  Inside  this  membrane,  it  is 
often  possible  to  distinguish  one  or  more  of  the  aforesaid 
nucleoli,  lying  in  a  more  fluid  material  often  called  the  "nuclear 
sap."  About  these  nucleoli  and  bodies  more  or  less  like  them, 
about  the  reasons  for  their  variable  number  and  form,  very  little 
that  is  certain  can  be  said.  Much  more  important  is  the 
essential  constituent  of  the  nucleus,  a  system  of  strands,  coils> 
or  loops,  which  stain  deeply  with  various  dyes,  and  are  there- 
fore known  as  the  chromatin  elements.  In  contrast  thereto,  the 
less  stainable  and  less  essential  constituents  of  the  nucleus  are 
distinguished  as  achromatin. 

The  chromatin  elements  in  the  resting  nucleus  are  oftenest 
arranged  in  a  manifold  coil,  like  a  disordered  ball  of  twine, 
while  in  other  cases  they  appear  rather  as  a  living  network. 
One  thing  about  them  seems  very  certain,  and  that  is  that  they 
are  in  no  disorder,  but  really  preserve  a  very  thorough  definite- 
ness.  Whether  the  coil  be  continuous,  as  Van  Beneden  and 
others  describe,  or  interrupted,  as  Boveri  and  others  maintain, 
is  subsidiary  to  the  more  striking  fact,  that  in  the  state  of  activity 
the  number  and  disposition  of  the  dislocated  or  loosened  parts 
of  the  coil  remain  definite  and  orderly,  and  that  their  behaviour 
is  so  like  that  of  minute  independent  individualities  that  any 
rough-and-ready  account  of  the  mechanics  of  cell  division  must 
at  once  be  ruled  out  of  court.  It  is  within  the  chromatin  sub- 
stance too  that  the  germ-plasma,  on  which  Weismann  and  others 
have  so  much  insisted,  has  its  seat. 

§  2.  Growth  of  the  Ovum. — When  the  ovum  is  very  young, 
it  very  generally  presents  the  features  of  an  amoeboid  cell.  In 
some  cases  this  phase  persists  for  a  longer  time,  as  in  the  ovum 
of  hydra,  which  in  all  essentials  is  comparable  to  an  amoeba. 
Even  in  the  simplest  animals,  however,   the  amoeboid  phase 


lOO  THE    EVOLUTION    OF    SEX. 

constantly  shows  a  tendency  to  pass  into  greater  quiescence,  to 
become  in  fact  more  or  less  encysted.  So  is  it  with  ova,  which 
though  at  first  often  resembling  various  forms  of  amoeboid  cells, 
tend  more  or  less  quickly  to  pass  into  the  encysted  phase. 
The  protoplasm  no  longer  flows  out  in  irregular  ever-changing 
processes,  but  is  gathered  up  into  a  sphere,  rounded  off,  and 
surrounded  by  a  more  or  less  definite  envelope.  This  transition 
from  a  state  of  relative  equilibrium  between  activity  and  pas- 
sivity, to  one  in  which  passivity  undoubtedly  preponderates,  is 
associated  with  an  increase  of  nutriment  and  reserve-products. 
The  ovum  feeds,  becomes  heavy  with  stored  capital,  becomes 
less  active,  and  more  encysted  in  consequence. 

§  3.  Yolk. — The  essential  part  of  an  egg-cell  is  always  small, 
though  even  in  this  there  are  great  differences.  The  nucleus, 
for  instance,  in  the  large  eggs  of  amphibians,  reptiles,  and 
birds,  may  be  detected  with  the  unaided  eye ;  while  in  other 
cases,  such  as  sponges,  the  entire  ovum  is  very  minute.  Yet 
every  one  knows  that  eggs  vary  enormously  in  size.  The  egg 
of  a  skate  is  very  much  larger  than  the  egg  of  a  salmon  ;  and  the 
egg-shell  of  the  extinct  giant  bird  of  Madagascar  (^Tvpyornis)  is 
big  enough  to  hold  the  contents  of  one  hundred  and  fifty  hens' 
eggs.  Similarly  the  contrast  between  the  eggs  of  ostrich  and 
humming-bird  is,  as  one  would  expect,  extremely  striking. 
Yet  the  eggs  of  whales  are  "not  larger  than  fern-seed,"  and  the 
same  is  true  for  most  mammals,  except  the  very  lowest.  The 
differences  in  size,  when  very  striking,  are  due  not  so  much  to 
any  marked  disproportion  in  the  essential  parts  of  the  ova,  but 
to  certain  extrinsic  additions.  The  most  important  of  these  is 
the  yolk,  which  serves  as  nutritive  capital  for  the  embryo  or 
young  animal.  Besides  the  yolk,  we  have  also  to  take  into 
account  the  frequent  pigment,  so  familiar  in  frog  spawn,  the 
albumen  well  seen  in  the  white  of  birds'  eggs,  various  forms  of 
protective  and  attaching  viscid  material,  and,  lastly,  more  or 
less  elaborate  egg  envelopes  or  shells.  The  most  important, 
however,  is  the  yolk,  and  in  regard  to  its  origin  and  dis})Osition 
a  little  must  be  said. 

The  egg  has  its  nutritive  capital  increased  in  three  different 
ways  : — {a.)  Very  generally  it  feeds  on  the  nutritive  elements  in 
the  general  lymph  or  vascular  fluid  of  body,  {b.)  At  the  same 
time,  or  in  another  case,  it  avails  itself  of  the  debris  of  surround- 
ing cells.  In  many  instances,  e.g..,  in  the  minute  ovary  of 
hydra,  or  in  the  ovarian  tubes  of  insects,  the  ovum  is  but  the 


THE    EGG-CELL    OR    OVUM. 


lOT 


surviving  competitor  among  a  crowd  of  surrounding  cells,  which 
to  start  with  were  all  potential  ova.  (c.)  In  the  third  place,  and 
this  is  the  rarest  form,  the  egg-cell  acquires  a  store  of  food- 
material  from  a  special  yolk  gland,  as  in  many  of  the  lower 
"worms."  But  we  have  already  pointed  out  that  this  yolk- 
gland  is  usually  interpreted  as  a  degenerate  portion  of  the 
essential  organ. 


B 


J) 


The  relation  between  the  disposition  of  the  yolk  and  the  mode 
of  segmentation  : — A,  diffuse  yolk,  e.^.,  sponge  ;  B,  polar, 
c.£:,  frog  ;  C,  central  yolk,  e  ^^.,  crayfish  ;  D,  predomin- 
ant, e.£:,  bird  : — A',  lotal  and  equal  segmentation  ;  B', 
total  and  unequal;  C,  peripheral ;  D',  partial  segmentation. 

The  yolk,  gained  in  the  above  ways,  is  more  or  less  readily 
distinguished  from  what  is  often  called  the  formative  protoplasm. 
Out  of  the  latter  the  embryo  is  built  up,  while  the  yolk  has 
for  the  most  part  only  a  secondary  and  nutritive  7v/e.  We 
cannot,  of  course,  enter  here  into  the   difficult  embryological 


I02  THE    EVOLUTION    OF    SEX. 

question  as  to  the  extent  in  which  the  yolk  ever  shares  in 
directly  contributing  to  embryonic  structures.  The  possibility 
of  distinguishing  between  formative  protoplasm  and  the  nutritive 
material,  depends  on  the  quantity  of  the  latter  that  is  present, 
and  on  the  way  in  which  it  is  disposed,  {a.)  When  there  is 
not  much  of  it,  as  in  the  small  ova  of  mammals  and  many 
invertebrates,  the  yolk  material  is  diffusely  distributed.  Then 
the  ovum  undergoes  complete  segmentation,  {b.)  In  the  frog's 
ovum,  on  the  other  hand,  there  is  a  large  proportion  of  yolk, 
which  has  especially  accumulated  in  the  lower  hemisphere  of 
the  cell,  while  the  darker  half  includes  the  truly  formative  pro- 
toplasm. In  this  case  too  the  egg  divides  as  a  whole,  but  the 
divisions  go  on  much  more  rapidly  in  the  upper  hemisphere, 
and  it  is  there  that  the  embryo  is  really  formed,  {c.)  A  dis- 
tinct mode  of  yolk  arrangement  occurs  in  arthropods  (crusta- 
ceans, insects,  <S:c.),  where  the  centre,  not  a  pole,  of  the  ovum 
is  occupied  by  the  nutritive  material.  In  this  case  the  forma- 
tive protoplasm  divides  round  about  the  nutrient  core,  (d.)  In 
the  majority  of  fishes,  in  reptiles,  and  in  birds,  the  eggs  show  a 
much  more  marked  polar  accumulation  of  yolk.  On  the  top 
of  a  large  mass  of  nutritive  material,  the  specifically  lighter 
formative  protoplasm  lies  like  a  tiny  drop,  and  in  those  cases 
the  division  of  the  ovum  is  very  partial, — that  is,  it  is  mainly 
restricted  to  the  upper  formative  region.  It  is  thus  to  be  noted 
that  the  quantity  of  yolk  present,  and  its  diffuse,  polar,  or 
central  arrangement,  are  associated  with  striking  differences  in 
the  degree  and  symmetry  of  the  segmentation. 

§  4.  Composile  Ova.  — We  have  emphasised  the  fact  that  the  ovum  must 
be  regarded  as  a  single  cell.  To  this  a  definite  but  pedantic  objection 
has  been  raised.  In  some  parasitic  flat  worms  there  occur  what  have  been 
called  compound  ova.  A  minute  single  cell  arises,  as  usual,  in  the  ovary, 
but  in  the  course  of  its  somewhat  intricate  history  this  becomes  associated 
with  several  nutrient  cells  derived  from  the  yolk-gland.  These  sur- 
round the  original  ovum,  so  that  the  whole  now  consists  of  several  cells. 
But  It  must  be  noticed  that  only  the  central  cell — the  ovum  proper — is 
fertilised,  and  that  it  contains  all  the  formative  protoplasm.  Those 
that  surround  it  are  wholly  nutritive  ;  they  eventually  break  up,  and  are 
absorbed. 

In  other  cases,  especially  in  insects,  the  ovum  grows  rich  at  the  expense 
of  neighbouring  cells,  which  are  sacrificed  to  its  nutritive  equipment,  l^ut  it 
is  evident  enough  that  a  cell  remains  a  cell,  however  many  of  its  neighbours 
it  may  happen  to  absorb. 

§  5.  Ei:;;^  Envelopes. — The  ovum  starts  as  a  naked  cell,  but  generally 
becomes  furnished  with  ensheathing  envelopes.  The  exact  history  of  the 
egg-membranes  and   sheaths  is  a  very  complex  matter.     Only  the  most 


THE    EGG-CELL    OR    OVUM.  I03 

general  facts  can  here  be  stated.  The  envelopes  may  be  derived  {a)  from 
the  ovum  itself,  {/>)  from  surrounding  cells,  {c)  from  the  secretion  of  special 
glands. 

{a.)  Just  as  a  protozoon  often  exhibits  distinct  outer  and  inner  zones, 
distinguished  by  minor  physical  and  chemical  peculiarities,  so  it  is  with  the 
ovum.  What  are  called  yolk  or  vitelline  membranes  are  generally  pro- 
duced by  the  ovum  itself.  Furthermore,  the  outer  protoplasm  often  forms 
a  distinct  firm  zone,  known  as  the  Zona  pellucida.  This  may  be  traversed 
by  fine  radiating  pores  establishing  nutritive  communication  with  the 
exterior,  and  is  then  known  as  the  Zona  radiata.  A  special  aperture  or 
inicropyle  is  sometimes  present,  through  which  the  sperm  enters,  or  nutri- 
tive supply  is  sustained. 

{b. )  The  ovum,  in  its  young  stages,  is  very  frequently  seen  surrounded 
by  a  circle  of  small  cells,  which  form  what  is  called  a  follicle.  These 
may  produce  a  membrane  or  a  glairy  investment.  According  to  some 
investigators  {e.g..  Will),  the  follicular  cells  sometimes  arise  from  within  the 
ovum,  as  the  result  of  an  early  activity  in  the  nucleus.  This  view,  however, 
cannot  be  said  to  be  confirmed. 

{c.)  As  the  ovum  ripens,  and  passes  from  the  ovary  into  the  duct,  it 
often  becomes  surrounded  by  gelatinous,  horny,  limy,  and  other  invest- 
ments. In  most  cases,  it  necessarily  follows  that  the  egg  has  first  been 
fertilised.  The  investments  are  usually  referable  to  the  activity  of  the 
walls  of  the  oviduct  or  uterus,  though  sometimes  there  are  special  shell- 
glands,  and  the  like.  The  chitinous  cases  of  some  insect  ova,  the  horny 
mermaids'  purses  of  many  gristly  fishes,  the  more  or  less  limy  egg- 
envelopes  of  reptiles,  the  firm  limy  egg-shells  of  birds,  so  often  stained  with 
pigments,  afford  good  illustrations  of  these  secondary  investments.  Quite 
distinct  are  cocoons,  such  as  those  of  earthworm  and  leech,  which  surround 
several  eggs,  and  are  produced  from  the  skin  of  the  animal. 

§  6.  Birds'  Eggs. — The  student  may  be  fitly  directed  to  the 
egg  of  the  fowl,  or  of  some  other  bird,  for  a  convenient  concrete 
illustration  of  many  facts.  There  he  will  see  the  great  mass  of 
yolk,  of  two  kinds,  yellow  and  white,  and  on  the  top  of  this  the 
minute  area  of  formative  protoplasm.  It  was  on  this,  as  it 
gradually  revealed  the  cloudy  outlines  of  the  embryo  chick, 
that  the  Greeks  looked  with  naive  unaided  eyes.  Here  it  was 
that  Aldrovandus,  Harvey,  Malpighi,  Haller,  and  the  early 
embryologists,  with  clear  vision,  saw  almost  as  much  as  their 
appliances  would  permit.  It  was  this  which,  in  its  primitive 
simplicity,  impressed  Wolff  with  the  reality  of  epigenesis  ;  and  it  is 
this  that  the  observers  of  to-day  look  down  upon  through  their 
embryoscopes,  or  cut  sections  of  with  their  microtomes.  Then 
round  about  all  is  the  secondary  investment  of  "  white  of  egg" 
or  albumen ;  round  this  a  shell  membrane,  between  the  two 
layers  of  which  the  little  air-chamber  is  formed  ;  and  finally,  the 
hard  but  porous  limy  shell.  There  arises  the  difficult  problem 
of  the  origin  of  the  shell,  in  regard  to  which  it  is  to  be  noted  that 


I04  THE    EVOLUTION    OF    SEX. 

Mr  Irvine,  of  Granton,  has  recently  shown  that  fowls  kept  with 
access  to  no  carbonate,  but  only  to  other  salts  of  lime,  can  still 
form  a  normal  shell.  This  still  consists  of  carbonate  of  lime, 
and  is  as  firm  as  usual,  demonstrating,  like  the  same  investi- 
gator's experiments  on  crabs,  that  animals  possess  no  little 
power  of  changing  one  salt  of  lime  into  another.  Then, 
in  the  eggs  of  other  birds,  the  import  of  the  seven  or 
more  pigments  which  produce  the  marvellous  variety  and 
beauty  comes  into  question.  Sorby  has  shown  that  they 
are  related  to  the  pigments  of  blood  and  bile ;  but  what 
they  exactly  mean  no  one  yet  knows.  Wider  still,  the 
problem  arises  of  how  this  coloration  is  so  often  protective ; 
and  whether  Lucas  is  right  in  supposing,  that  the  colour  of 
the  surroundings  can  actually  influence  the  deposition  of  pig- 
ment, by  acting  on  the  nervous  system  of  the  mother  bird. 
Or  again,  there  is  the  curious  fact,  that  the  size  of  the  egg  is 
often  much  out  of  proportion  to  the  size  of  the  bird,  and  the 
question  arises  as  to  how  far  this  can  be  interpreted  as  the 
result  of  the  more  or  less  anabolic  and  sluggish  constitution. 

§  7.  Chemistry  of  the  Egg. — Every  one  knows  that  the  eggs  of  birds  form 
highly  nutritious  diet.  As  the  egg  contains  nourishment  for  the  young  bird 
for  a  considerable  time,  it  must,  like  milk,  contain  all  the  essentials  of  food. 
The  results  of  a  recent  analysis  of  the  fowl's  egg  may  be  taken  as  a 
sample. 

The  germinal  or  formative  disc  consists  chiefly  of  albuminoid  bodies, 
apparently  of  the  globulin  group,  plus  smaller  quantities  of  lecithin  and 
the  like.  The  subtle  protoplasm  itself,  it  need  hardly  be  said,  defies 
analysis. 

In  the  yolk  there  are  firm  fats  (tripalmiiin,  probably  plus  a  little 
stearine),  and  a  fluid  oil  or  glyceride.  P'atty  acids  develop  during  hatch- 
ing. A  relatively  large  quantity  of  lime  is  present,  probably,  for  the  most 
part,  as  calcium  albuminate.  In  the  white  of  egg  there  are  true  albumins, 
also  globulins,  and  the  quantity  of  peptones  increases  with  the  age  of  the 
egg.  During  development  the  embryo  becomes  richer  in  mineral  matters, 
fat,  and  albumen,  and  the  dry  substance  of  the  whole  contents  of  the  egg 
diminishes  considerably. 

The  yolk  of  many  different  kinds  of  ova  has  been  analysed,  and  the 
component  substances  distinguished  as  IcJithin  (fishes),  Emydin  (tortoise), 
and  the  like.  More  important  were  the  discoveries  of  cholesterm,  vitelltji, 
mtcleiii,  lect'thi7i,  and,  in  association  with  the  latter,  neuriti.  As  we  cannot 
here  enter  into  the  physiological  import  of  such  substances,  it  is  enough  to 
say  that  the  nutritive  material  in  ova  usually  consists  of  a  mixture  of  com- 
plex, unstable,  and  highly  nutritive  substances. 

§  8.  Mahi7'atio7i  of  the  Ovum. — When  the  egg-cell  has 
attained  its  mature  size,  a  more  or  less  enigmatical  occurrence 
takes  place.     The  nucleus,  hitherto  generally  central,  moves  to 


THE    EGG-CELL    OR    OVUM.  I05 

the  pole,  alters  considerably  in  its  structure,  and  divides.  A 
minute  cell,  with  half  of  the  nucleus,  and  a  small  amount  of 
protoplasm,  is  given  off  Not  long  after,  the  nucleus  remaining 
within  the  ovum  repeats  the  process,  and  another  tiny  cell  is 
expelled.  This  process,  which  the  majority  of  investigators 
regard  as  one  of  normal  cell-division  or  cell-budding,  is  known 
as  the  extrusion  of  the  polar  globules.  Of  general,  and  probably 
of  universal  occurrence,  it  has  been  but  rarely  observed  in 
fishes  and  amphibians,  and  not  as  yet  demonstrated  in  reptiles 
or  birds.  It  was  for  long  thought  to  be  absent  in  arthropods, 
but  the  researches  of  Weismann,  Blochmann,  and  others,  have 
shown  that  this  is  not  the  case.  An  interesting  peculiarity, 
which  we  shall  afterwards  notice,  has  been  demonstrated  by 
Weismann  in  regard  to  parthenogenetic  ova.  There  is  con- 
siderable diversity  as  to  the  exact  time  at  which  the  extrusion 
occurs ;  generally,  however,  it  precedes  the  entrance  of  the 
fertilising  sperm.  The  minute  extruded  cells  never  have  any 
history,  though  they  occasionally  linger  for  a  considerable  time 
on  the  outskirts  of  the  ovum.  As  an  exception,  they  have  been 
seen  themselves  to  divide,  and,  with  equal  rarity,  a  misguided 
spermatozoon  has  been  observed  to  penetrate  them.  Usually, 
however,  they  simply  dwindle  away.  The  remaining  female 
nucleus  of  the  ovum  is  now  ready  to  unite  with  the  male 
nucleus  of  the  spermatozoon.  By  the  twofold  division  just 
described  it  has  been  considerably  reduced  in  size,  though  not 
a  whit  in  complexity,  or  in  the  number  of  its  chromatin  elements. 
At  this  point,  awaiting  the  essential  moment  of  fertilisation,  we 
shall  for  the  present  leave  it. 

Within  the  last  two  years,  Weismann,  assisted  by  C. 
Ischikawa,  has  demonstrated  an  exceedingly  interesting  fact  in 
regard  to  polar  globule  extrusion  in  parthenogenetic  ova. 
Instead  of  the  two  polar  globules  which  are  usually  extruded, 
parthenogenetic  ova  were  shown  to  form  only  one.  This  was 
demonstrated  in  a  variety  of  cases, — in  water-fleas  (daphnids 
and  ostracodes)  and  rotifers,^and  is  believed  by  this  eminent 
authority  to  be  a  general  fact.  Blochmann,  who  has  been 
successful  in  demonstrating  polar  globules  in  several  orders  of 
insects,  has  also  observed  that  in  the  parthenogenetic  ova  of 
the  plant-louse  or  aphis,  only  one  polar  globule  was  formed, 
while  the  eggs,  which  only  developed  after  fertilisation,  two 
occurred  as  usual.  To  these  facts  we  must  afterwards  recur 
in  connection  with  parthenogenesis. 


Io6  THE    EVOLUTION    OF    SEX. 

§  9.  Theories  of  the  Polar  Globules.  —  The  polar  globules  appear  to 
have  been  first  observed  in  1848  by  Fr.  Miiller  and  Loven,  but  it  is  only 
within  recent  years  that  much  has  been  made  of  them.  Thanks  to  the 
masterly  researches  of  Butschli  and  Hertwig,  Giard,  Fol,  and  others,  it 
became  possible  to  interpret  the  extrusion  as  a  case  of  cell-division  or 
budding.  More  recently,  Van  Beneden,  whose  monograph  on  the  ovum  of 
the  threadworm  {Ascaris)  will  remain  one  of  the  classics  in  this  department 
of  research,  has  raised  a  protest  against  regarding  the  extrusion  as  a 
normal  cell-division.  The  details  of  the  process,  as  interpreted  by  him, 
seemed  to  mark  out  the  extrusion  as  something  unique.  The  latest  results 
of  Boveri,  Zacharias,  and  others,  however,  confirm  the  older  view,  that  the 
process  is  essentially  one  of  normal  cell-division. 

But  while  this  structural  fact  may  be  regarded  as  certain,  there  is  no 
unanimity  as  to  what  the  process  means.  The  chief  opinions  on  this  subject, 
only  a  mere  outline  of  which  can  be  given,  are  three,  not  including  a  number 
of  suggestions  according  to  which  the  extrusion  of  the  globules  is  a  kind  of 
"  excretion  "  of  the  ovum,  or  a  "  rejuvenescence  "  of  the  nucleus. 

{a)  According  to  some,  the  egg-cell  is  in  a  sense  hermaphrodite,  and  the 
polar-globule  formation  is  an  extrusion  of  the  male  element.  Balfour  ex- 
pressed his  view  in  somewhat  teleological  language: — "I  would  suggest 
that  in  the  formation  of  the  polar  cells,  part  of  the  constituents  of  the 
germinal  vesicle,  which  are  requisite  for  its  functions  as  a  complete  and 
independent  nucleus,  is  removed  to  make  room  for  the  supply  of  the  neces- 
sary parts  to  it  again  by  the  spermatic  nucleus.  ...  I  will  venture  to 
add  the  further  suggestion,  that  the  function  of  forming  polar  cells  has  been 
acquired  by  the  ovum  for  the  express  purpose  of  preventing  parthenogenesis." 
To  this  it  must  now  be  pointed  out,  that  so  far  as  one  polar  globule  is  con- 
cerned, extrusion  does  not  prevent  parthenogenesis.  This  view  seems, 
according  to  Brooks,  to  have  been  first  advanced  by  M'Crady.  It  has 
been  most  carefully  elaborated  by  Minot.  According  to  Minot,  "in  the 
cells  proper,  both  sexes  are  potentially  present ;  to  produce  sexual  elements 
the  cell  divides  into  its  parts  ;  in  the  case  of  the  egg-cell,  the  male  polar 
glol)ules  are  cast  off,  leaving  the  female  ovum."  In  parthenogenetic  ova,  he 
supposes  that  enough  male  element  is  retained,  since  only  one  polar  globule 
appears  to  be  formed.  Van  Beneden,  whose  opinion  is  entitled  to  great 
weight,  also  inclines  to  regard  the  polar  globules  as  male  extrusions. 

Sabatier  distinguishes,  besides  true  polar  globules,  other  extrusions,  and 
believes  the  eliminated  parts  to  be  male  elements.  His  views  are  connected 
with  an  elaborate  theory  of  polarities,  according  to  which,  for  instance,  the 
peripheral  extrusions  are  male,  while  central  cores  (in  the  development  of 
sperms)  are  female  residues. 

(/<)  A  very  different  view — morphological  rather  than  physiological — 
has  been  maintained  by  Biitschli,  Whitman,  and  others.  The  formation  of 
polar  globules  is  an  atavistic  reminiscence  of  the  primitive  parthenogenesis. 
Just  as  the  mother  sperm-cell  or  spermatogonium,  which  corresponds  in 
the  male  to  the  ovum  in  the  female,  divides  up  into  what  form  spermatozoa, 
so  the  ovum  retains  a  slight  power  of  division.  Yet  parthenogenetic  ova, 
so  fhr  as  polar-globules  are  concerned,  show  this  least,  nor  can  we  well 
conceive  an  atavism  so  universally  jiresent  without  some  important  physio- 
logical necessity  directly  behind  it.  To  liiitschli's  view,  however,  such  an 
authority  as  Hertwig  inclines,  and  Boveri  likewise  interjirets  the  polar 
globules  as  "  abortive  ova." 


THE    EGG-CELL    OR    OVUM.  IO7 

(c)  Weismann's  view  is  different  from  either  of  the  above,  though 
nearer  the  first.  He  distinguishes  in  the  nucleus  of  the  ovum  two  kinds  of 
plasma, — (i)  the  ovogenetic  or  histogenetic  substance,  which  enables  the 
ovum  to  accumulate  yolk,  secrete  membranes,  and  the  like  ;  and  (2)  the 
germ-plasma,  which  enables  the  ovum  to  develop  into  an  embryo.  When 
the  ovum  is  mature,  the  ovogenetic  substance  has  served  its  turn  ;  it  is 
henceforth  only  an  encumbrance  ;  it  is  extruded  as  the  first  polar  globule. 
This  is  all  that  is  extruded  in  parthenogenetic  ova.  The  second  extrusion 
is  a  reduction  of  the  germ-plasma  itself  by  half,  and  the  same  must  occur 
in  the  male  germ  cell  too.  What  is  lost  in  the  second  polar  globule  is 
supplied  by  the  fertilising  sperm.  The  beginning  of  development  depends 
upon  the  presence  of  a  definite  quantity  of  germ-plasma.  This  the  normal 
egg  attains  by  first  losing  half  and  then  regaining  it,  while  the  partheno- 
genetic egg  attains  the  same  result  by  never  losing  any  at  all. 

In  this  too  there  is  much  hypothesis.  The  two  kinds  of  nuclear  plasma, 
the  difference  between  the  two  polar  globules,  the  necessity  for  a  definite 
quantity  before  development  begin,  are  all  assumptions.  Nor  is  it  at  all 
evident  how  the  advantage  of  fertilisation  (as  a  source  of  progressive  change 
and  so  on)  could  operate,  so  as  to  induce  the  ovum  to  go  through  the  circuitous 
process  of  losing  half  its  "  germ-plasma,"  and  then  gaining  it  again. 

((/)  It  appears  simpler  to  us  to  suppose  that  the  ovum,  like  any  other  cell, 
tends  to  divide  or  bud  at  the  limit  of  growth,  a  view  in  no  way  inconsistent 
with  regarding  the  process  as  an  extrusion  of  male  elements.  The  precise 
homologies  of  the  process  will  be  clearer  on  reference  to  the  diagram  at 
page  1 14. 


Io8  THE    EVOLUTION    OF    SEX. 


SUMMARY. 

1.  The  ovum  presents  all  the  essential  features  of  a  cell  ;  its  substance 
and  nucleus  described.  The  chromatin-elements  of  the  latter  are  the 
essential  parts. 

2.  The  ovum  usually  grows  from  an  amoeboid  to  an  encysted  phase,  with 
increase  of  nutrition  and  size. 

3.  The  yolk  is  derived  from  the  vascular  fluid,  or  surrounding  cells,  or 
special  glands,  and  is  present  in  varying  quantity  and  disposition.  If  little, 
it  is  diffuse  ;  if  much,  it  is  polar  or  central  ;  and  the  different  modes  of  egg- 
division  are  associated  with  this. 

4.  In  some  cases  the  ovum  is  surrounded  by  a  number  of  nutritive  cells 
(composite  ova),  and  often  becomes  what  it  is  by  preying  upon  its 
neighbours.     This  hardly  aff'ects  its  unicellular  character. 

5.  Egg-envelopes  are  produced  from  the  ovum  itself  (>?.,;?".,  vitelline 
membrane),  or  from  surrounding  cells  (follicular  sheath),  or  from  special 
glands  (the  outside  shell). 

6.  Bird's  egg  noted  as  a  concrete  illustration  of  facts  and  problems. 

7.  The  egg,  so  far  as  its  nutritive  material  is  concerned,  includes  a 
mixture  of  complex,  unstable,  highly  nutritive  substances. 

8.  The  maturation  of  the  ovum  is  usually  associated  with  a  double  cell- 
division  or  budding,  known  as  the  extrusion  of  polar  globules.  In 
parthenogenetic  ova  only  one  seems  to  occur. 

9.  This  polar  globule  formation  has  been  interpreted  variously  : — {a) 
As  an  extrusion  of  male  elements  (Minot,  Balfour,  Van  Beneden);  (/')  as  an 
atavistic  occurrence  of  cell-division  (Biitschli,  Whitman,  Hertwig,  &c.); 
(c)  by  Weismann's  more  complex  hypothesis.  It  seems  to  be  a  case  of  cell- 
division  at  the  limit  of  growth. 


LITERATURE. 

Balfour,  F.  M.  —  0/>.  cit. 

Van  Beneden,  E. — Recherches  sur  la  Fecondation.     Arch,  de  Biologic, 

IV.,  1883. 
Carnoy. — La  Cellule  II.,  1886,  &c. 
Geddes,  p.  —  Op.  cit. 
Haddon,  a.  C. — Op.  cit.        * 
Hensen,  V.  —  Op.  cit. 
Hertwig,  O. — Op.  cit. 
Haiche'it  Jackson. — Introduction  to  his  edition  of  Rolleston's  Forms 

of  Animal  Life. 
M'Kendrick,  J.  G. — On  the  Modern  Cell  Theory,  6cc.     Proc.  Phil.  Soc. 

Glasgow,  XIX.,  1888. 
Minot,  C.  S. — American  Naturalist,  XIV.,  1880. 
Thomson,  J.   A. — Recent    Researches   on   Oogenesis.     Quarterly  Journ. 

Micr.  Sci.,  XXVI.,  1886. 

Art.  Embryology,  Chambers's  Encyclopaxlia. 

Weismann,  a. — Die  Continuitiit  des  Keimplasmas.     Jena,  1885. 

Die  Bedcutung  der  sexuellen  Fortpflanzung.     Jena,  1886. 

And  other  papers  recently  translated,  "  Heredity.''     Oxford,  1889. 


CHAPTER    IX. 

The  Male-Cell  or  Spermatozoon. 

§  I.  TJie  Genei'al  Contrast  between  Ovum  and  Spermatozoon. 
— Just  as  the  ovum,  large,  well  nourished,  and  passive,  is  a 
cellular  expression  of  female  characteristics,  so  the  smaller  size, 
less  nutritive  habit,  and  predominant  activities  of  the  male  are 
summed  up  in  the  sperm.  As  the  ovum  is  usually  one  of  the 
largest,  the  sperm  is  one  of  the  smallest  of  cells.  The  yolk  or 
food-capital,  and  encysting  membranes,  which  are  often  so  pro- 
minent in  the  former,  are  as  conspicuously  absent  in  the  latter. 
The  contrast,  though  less  accented,  is  still  quite  discernible  in 
plants.  In  fact,  the  two  kinds  of  cells  are  just  as  widely  op- 
posed in  their  general  features,  as  they  are  fundamentally  com- 
plementary in  their  history.  Before  this  opposition  and  comple- 
mentariness  can  be  fully  understood,  however,  we  must  briefly 
sum  up  the  characters  and  history  of  the  male  elements. 

§  2.  History  of  Discovery. — In  1677,  one  of  Leeuwenhoek's  students, 
Hamm  by  name,  called  his  master's  attention  to  the  minute  elements 
actively  moving;  in  the  male  fluid.  Leeuwenhoek,  who  some  years  pre- 
viously for  the  first  time  observed  what  we  now  know  as  unicellular 
organisms,  was  at  once  impressed  by  the  import  of  the  marvellously  active 
male  units.  Almost  too  much  impressed,  in  fact,  for  he  interpreted  them 
as  minute  preformed  germs,  which  only  required  to  be  nourished  by  the 
ovum  to  unfold  into  embryos.  Thus  the  unfortunate  aberration,  already 
noted  as  the  doctrine  of  the  animalculists,  had  its  origin.  For  long  no 
progress  whatever  was  made  ;  some  naturalists,  like  Vallisneri,  depreciat- 
ing the  import  of  the  sperms  altogether,  and  regarding  them  as  worms 
which  hindered  the  coagulation  of  the  seminal  fluid;  others  going  to  the 
opposite  extreme,  and  regarding  them  as  nests  of  germs.  Thus  Haller  at 
first  considered  them  to  be  what  Leeuwenhoek  had  suggested,  but  after- 
wards admitted  them  merely  as  nativi  Jiospites  seniiiiis.  In  1835,  even  Von 
IJaer  was  inclined  to  interpret  them  as  minute  parasites  peculiar  to  the 
male  fluid  ;  and  if  the  curious  student  will  turn  up  the  article  Eiitozoa  in 
Todd's  Cyclopitdia  of  Anatoiiy  and  Physiology.,  of  about  the  same  date,  he 
will  find  that  the  veteran  Owen  includes  the  spermatozoa  under  that  strange 
heading.  The  very  name  spermatozoon  recalls  the  view  which  so  long 
prevailed. 

In  1837,  R.  Wagner  emphasised  their  constancy  in  all  the  sexually 
mature  males    which    he    examined,   and   their  absence   in   infertile  male 


I  lO 


THE    EVOLUTION    OF    SEX. 


hybrids  ;  Von  Siebold  demonstrated  their  presence  in  many  of  the  lower 
animals  ;  and  lastly,  in  184.1,  KoUiker  made  one  of  his  many  important 
contributions  to  biology,  in  proving  that  the  sperms  had  a  cellular  origin 
in  the  testes. 

§  3.  Structure  of  the  Sperm. — The  sperm,  then,  is  a  cell. 
Though  some,  such  as  Kolliker,  have  inclined  to  regard  it 
rather  as  a  nucleus,  its  truly  cellular  character  may  be  regarded 
as  proven  beyond  dispute.  We  have,  as  in  the  ovum,  to  deal 
with  cell-substance  and  nucleus,  with  this  marked  difference, 
that  the  cell- substance  is  generally  reduced  to  a  minimum. 


"  Spermatic  Animalculi  "  of  the  Rabbit  and  the  Dog. 
— From  Buffon,  after  Leeuwenhoek. 

The  sperm  is  almost  always,  moreover,  a  cell  of  a  very 
definite  type  or  phase.  It  is  like  one  of  the  highly  motile 
Protozoa,  like  a  flagellate  infusorian.  Usually  it  consists  of  a 
minute  "  head,"  consisting  almost  entirely  of  nucleus,  and  of  a 
long  contractile  tail,  which,  working  behind  like  a  screw,  propels 
the  essential  "  head"  through  the  water  or  along  the  ducts.  Oc- 
casionally, as  the  diagram  shows,  there  is  a  departure  from  the 
predominant  phase  of  cell-life.  Thus  in  the  threadworm 
Ascaris,  the  sperm  has  a  blunt  pear-shaped  form,  and  exhibits 
slight  amoeboid  movements.  In  some  crustaceans  and  other 
arthropods,  the  cell  is  even  more  quiescent,  and  may  exhibit 
curious  forms  such  as  that  figured  for  the  crayfish.  The 
relatively  dormant  activity  may  however  wake  up,  and  the 
sperm  exhibit  active  amoeboid  movements.  Zacharias  has 
made  some  interesting  experiments,  showing  the  modifiability 
of  sperms  under  reagents  ;  thus,  in  a  little  crustacean  (Foly- 
p/ie/nus  pediculus),  he  first  caused  the  cylindrical  sperm  to  form 
amoeboid  processes,  and  afterwards  to  replace  these  by  what 
were  to  all  intents  and  purposes  cilia.  This  is  entirely  con- 
gruent with  other  experiments  and  observations  on  the  passage 
of  cells  from  one  phase  of  the  cell-cycle  to  another. 

The  progress  of  microscopic  technique  has  demonstrated  many  com- 
plexities in  the  sperm  as  well  as  in  the  ovum.     For  a  discussion  of  some  of 


THE    MALE-CELL    OR    SPERMATOZOON. 


Ill 


the  more  important  of  these,  the  reader  is  referred  to  the  Encyclopiidia 
Britaitnica^  article  Reproduction.  A  few  points  only  need  be  noticed  here. 
Thus  most  spermatozoa  exhibit  not  only  a  head  (almost  wholly  from  the 
nucleus  of  the  mother-cell),  and  a  mobile  tail  (from  the  substance  of  the 
mother-cell),  but  a  median  portion  connecting  these.  The  tail  is  not 
unfrequently,  as  in  salamander  and  man,  furnished  with  a  very  delicate 
undulating  or  vibratile  band.  Complexities  such  as  axial  filaments,  stria- 
tions,  and  the  like  abound.    In  a  few  cases,  as  in  the  threadworm,  the  sperm 


Spermatozoa  of  crayfish  («),  lobster  (/'),  crab  (c),  ascarid  (if), 
water-flea — moina  {e),  man  (_/"),  ray  (g),  rat  (//),  guinea-pig 
(z),  a  beetle — immature  stage  (/t),  sponge  (/). 

is  not  left  without  any  nutritive  capital,  but  furnished  with  this  in  the  form 
of  a  cap,  which  falls  off  before  the  essential  moment  of  fertilisation  arrives. 
Important  perhaps  is  the  observation,  mainly  due  to  Flemming,  that  the  head 
of  the  sperm  not  only  arises  from  the  nucleus  of  the  mother-cell,  but  almost 
wholly  consists  of  the  chromatin-elements  of  the  same. 

§  4.  Physiology  of  the  Spe7'matozoo?i. — A  few  facts  in  regard 
to  the  physiology  of  the  sperm  demand  notice,  {a)  It  is 
specialised  as  a  highly  active  cell ;  its  minimal  size,  the  usual 
absence  of  any  encumbering  nutritive  material,  the  contractility 
of  the  tail,  and  the  general  shape,  all  fit  it  for  characteristic 
mobility.  More  than  one  histologist  has  likened  it  to  a 
free  muscle-cell,  and  its  resemblance  to  a  flagellate  monad  has 
already  been  noted,  {b)  Furthermore,  the  sperm  has  very 
considerable  power  of  persistent  vitality.  Not  only  does  it 
often  remain  long  unexpelled  in  the  male  animal,  without  losing 
its  functions,  but  it  may  retain  its  fertilising  power  after  remain- 
ing for  weeks,  or  even  months,  in  the  female  organism.  In  the 
earthworm,  the  spermatozoa  pass  from  one  worm  to  another,  not 
directly  to  the  ova  nor  to  female  ducts,  but  to  be  stored  up  in 


112 


THE    EVOLUTION    OF    SEX. 


special  reservoirs  or  spermathecse.  So  it  is  with  many  animals. 
The  spermatozoa  received  by  the  queen  bee  during  her  single 
impregnation,  are  for  a  considerable  period — even  for  three 
years — used  in  fertilising  successive  sets  of  worker  and  queen 
ova.  Quite  unique,  however,  is  the  case  of  one  of  Sir  John 
Lubbock's  queen  ants,  which  laid  fertile  eggs  thirteen  years  after 
the  last  se.^ual  union  with  a  male.  The  spermatozoa  had  ap- 
parently persisted  all  that  time.  Hensen  cites  the  facts,  that  a 
hen  will  lay  fertilised  eggs  eighteen  days  after  the  removal  of 
the  cock;  and  that  in  bats,  spermatozoa  may  remain  alive  a 
whole  winter  in  the  uterus  of  the  female,  {c)  Remarkable  too, 
and  again  like  monads,  is  the  power  the  sperms  have  of  suc- 
cessfully resisting  great  deviations  from  the  normal  temperature. 
The  presence  of  acids  has  usually  a  paralysing  influence,  but 
alkaline  solutions  have,  on  the  whole,  the  opposite  result. 


Diagram  of  the  Development  of  Spermatozoa  (upper  line),  of  the  Maturation  and 
F^ertilisation  of  the  Ovum  (lower  line). 

a,  an  amoehoid  sex-cell ;  A,  ovum,  with  germinal  vesicle,  n  ;  B,  ovum  extruding  first 
polar  body,  /'  and  leaving  nucleus  reduced  by  half;  C,  extrusion  of  second 
polar  body, /'^,  nucleus  «-,  now  reduced  to  one-fourth  of  original,  i,  a  mother 
sperm-cell,  dividing  (2,  3)  into  immature  and  mature  spermatozoa  {x/>.). 

D,  the  entrance  of  a  spermatozoon  ;  E,  the  male  and  female  nuclei  s/>.  n  and  n" 
approach  one  another. 

§5.  Origin  of  the  Sperffis. — A  primitive  female  cell  in  the 
ovary  grows  in  bulk  and  nutriment,  and  remains  intact,  but  a 
primitive  male  cell  in  the  testis  undergoes  repeated  division 
into  secondary  cells,  which  either  themselves,  or  by  further 
division,  form  the  spe'rms.  For  the  last  twenty  years  the 
development  of  spermatozoa  has  been  the  subject  of  almost 
continuous  research  and  controversy,  and  the  all  too-abundant 
nomenclature  affords  a  suggestive  index  to  the  confusion  out 


THE    MALE-CELL    OR    SPERMATOZOON 


113 


of  which  the  subject  is  now  emerging.  In  a  general  way, 
the  i^rocess  is  simply  that  of  the  varied  segmentation  of  a 
mcther-sperm-cell,  and  the  occurrence  of  a  series  of  preparatory 
stages  before  the  sperm  is  finally  matured.  In  detail,  however, 
there  are  many  variations,  and  these  are  described  in  a  maze  of 
often  tautologous  and  ambiguous  terms,  such  as  spermatogonium, 
spermatoblast,  spermatospore,  si)ermatogemma,  spermatomere, 
spermosphere,  and  a  dozen  more. 

One  of  the  most  defensible  set  of  terms  is  that  used  l)y  Voigt  after 
Semper,  and  also  by  Von  la  Valette  St  George,  who  has  worked  per- 
sistently at  the  subject  for  over  twenty  years.  The  sperm  or  spermatozoon 
is  differentiated  from  an  immature  cell  or  spermatide,  this  is  modified  from 
or  descended  from  a  spermatocyte,  the  spermatocytes  result  from  the 
division  of  the  mother-sperm-cell  or  spermatogonium,  and  this  finally  is  a 
modified  forrn  or  a  descendant  of  the  primitive  sex-cell  or  male  ovule. 


-:^mrry_ 


3" 


c" 


'vf^^llf 


Comparison  of  Spermatogenesis  and  Ovum  Segmentation. 

Explanation.— The  first  line,  A-E,  exhibits  types  of  ovum  segmentation  :— A,  regular  morula  ; 
B,  unequal  segmentation,  t'.»-.,  in  some  Molluscs  ;  C,  centrolecithal  or  peripheral  type,  i'.^., 
in  a  shrimp  Peneus  ;  D,  partial  segmentation  ;  E,  the  same,  with  the  cells  less  markedly 
defined  off  from  the  yolk. 

In  the  next  two  lines  various  types  of  spermatogenesis  are  collated  with  the  above  to 
illustrate  the  parallelism  :— A'  and  A",  morula  type,  as  -in  Sponge,  Turbellarian,  Spider,  &c.  ; 
B'  and  B",  where  the  division  is  unequal,  and  one  large  nutritive  cell  is  seen  (Plagiostome 
fishes.  Von  la  Valette  St  George);  C  and  C",  after  Blomfield,  Jensen,  &c.,  showing 
central  cytophoral  or  blastophoral  nutritive  portion  ;  D'  and  D",  sperm-blastoderm,  with  a 
few  formative  cells  on  large  nutritive  blastophore,  after  Gilson,  &c.  ;  E'  and  E",  the  Fame, 
with  the  sperm  cells  less  definitely  separated  off,  after  Von  Ebner  and  his  followers. 

H 


114  THE    EVOLUTION    OF    SEX. 

Difficulties  become  thick,  however,  when  we  inquire  into  the  division 
of  the  niother-sperm-cell  or  spermatogonium,  and  it  is  here  that  the  observa- 
tions of  recognised  authorities  so  much  disagree.  Accepting  the  results  of 
competent  observers,  we  have  elsewhere  endeavoured  to  rationalise  and 
unify  the  conflicting  observations,  by  comparing  the  different  modes  of 
spermatogenesis  with  the  different  forms  of  ovum-segmentation.  It  has 
been  already  incidentally  noticed,  that  the  egg-cell  may  divide  wholly  and 
equally,  or  unecjually,  or  only  very  partially,  or  round  a  central  core. 
Just  in  the  same  way  the  mother-sperm-cells  may  divide  into  a  uniform 
ball  of  cells,  or  only  at  one  pole,  or  only  at  the  periphery  round  a  central 
residue.  Balfour  and  others  had  hinted  at  this  comparison  in  the  use  of 
terms  like  sperm-morula;  and  Herrmann  had  also  concluded,  "  that  the 
division  of  the  male  ovule  into  a  series  of  generations  of  daughter-cells,  is 
a  phenomenon  comparable  to  that  exhibited  by  the  ovum  in  the  formation 
of  the  blastoderm.  ...  It  seems  then  more  important  to  determine 
exactly  the  mechanism  of  division,  than  to  give  a  particular  name  to  each 
stage  of  segmentation." 

Although  this  interpretation  of  spermatogenesis  by  collating  it  with 
ovum-segmentation  appears  to  Minot  "a  fanciful  comparison,"  in  favour  of 
which  he  is  "unable  to  recognise  any  evidence,"  neither  the  initial 
homology  between  the  mother-sperm-cell  and  ovum  with  which  we  start, 
nor  the  striking  parallelism  between  the  modes  of  division  of  these 
homologues  seem  thereby  even  disputed,  much  less  shaken.  The  widely 
different  conditions  in  which  these  two  processes  occur,  and  their  very 
different  meaning  to  the  organism,  are  of  course  as  obvious  to  us  as  to  any  ; 
but  here,  as  elsewhere,  the  morphologist's  comparisons  are  strictly  inde- 
pendent of  the  approval  of  the  physiologist. 

§  6.   Further  CoiJipa7-ison  of  Ovum 

and  Sperm.— \i  is  often  said  that  the 

sperm  is  the  male  cell  which  corresponds 

to  the  ovum.      This  is  only  true  in  a 

certain  sense.     In  function  the  two  ele- 

j[  jij;  ments  are  indeed,  in  a  general  way,  of 

Diagrammatic  comparison— I.  female  ^1      equal  rank,  and  are  obviously  comple- 

and  male  a}  cell  formed    from    the      mentary.      But  even  in  this  respect,  the 

division  of  a  single  cell  in  the  de-      two  elements,  which  unite  in  equal  pro- 

velopment    of  the  hermaphrodite  re-  .•  •      ^r,  .•    i         .       r  r     ••!• 

productive    organs    of    the    worm     po/^ions  m  the  essential  act  of  feriihs- 
Sagitta;     II.    ovum   b'^   and   polar     ation,  are  not  exactly  sperm  and  ovum, 
body   a'-\    III.    stump   of  moth  er-      but  {«)  the  head  or  nucleus  of  the  sperm 
sperm-cell^  and  the  spermatozoon  a3.      ^^^id  (/;)  the  female  nucleus  doubly  re- 
duced by  the  extrusion  of  two  polar  globules.     The  accurate  structural  resem- 
blance or  homology  is  not  between  ovum  and  sperm,  but  between  ovum  and 
mother-sperm-cell.*     This  fact,  pointed  out  by  Reichert  in  1847,  corrobor- 
ated by  Von  la  Valette  St  George,  Nussbaum,  and  others,  is  fundamental  to  a 
clear  comparison  of  the  history  of  ovum  and  sperm,  and  is  postulated  as  an 
accepted  fact  in  the  rationale  of  spermatogenesis  suggested  in  this  chapter. 

Since  the  above  was  written,  Platner  has  in  a  remarkable  manner 
demonstrated  the  unity  between  the  division  of  the  ovum  in  extruding 
polar  globules  and  the  division  of  the  spermatocytes.  In  both  cases 
occurs  the  unique  phenomenon  of  a  second  nuclear  division  following  on 
the  heels  of  the  first  without  the  intervention  of  the  usual  resting  phase. 


THE    MALE-CELL    OR    SPERMATOZOON.  II5 

It  is  possible  to  follow  out  the  homology  into  even  further  detail ;  thus  the 
antithesis  seen  in  polar-globule  formation  may  be  fairly  collated  with  similar 
separations  occurring  in  spermatogenesis.  Van  Beneden  and  Julin,  in  their 
researches  in  oogenesis  and  spermatogenesis  in  Ascaj-is,  have  noted  the 
morphological  correspondence  of  the  polar  globules,  as  we  may  call  them,  of 
both  ovum  and  sperm.  Again  we  have  a  recent  micro-chemical  demonstration 
of  the  similar  staining  reactions  of  polar  globules  in  ova,  and  the  correspond- 
ing remnant  of  the  parent  cell  in  spermatogenesis.  In  the  differentiation  of 
the  reproductive  cells  in  plants,  both  higher  and  lower,  similar  extrusions 
are  to  be  observed.  Of  this  Strasburger  has  given  numerous  illustrations, 
crowned  by  his  own  demonstration,  that  the  nucleus  of  the  pollen  grain,  in 
its  germination  upon  the  stigma,  separates  into  a  vegetative,  relatively 
unimportant,  and  a  generative  or  essential  nucleus.  Even  in  Protozoa, 
Biochmann  and  others  have  found  analogues.  A  process  so  general  is 
capable  of  a  unified  explanation,  more  specific  than  that  of  simply  referring 
the  matter  to  the  mysterious  necessities  of  cellular  physiology.  Just  as  in 
the  development  of  the  "  worm  "  Sao/Ua  a  single  cell  divides  into  two, 
which  become  the  starting-points  of  male  and  female  organs  respectively, 
so  the  cell  divisions  above  alluded  to  express  antitheses  between  more 
katabolic  and  more  anabolic  protoplasmic  constituents. 

§  7.  Chetnistry  of  tJie  Sperm. — Comparatively  little  has  been  done  in 
regard  to  the  chemistry  of  the  male  elements  in  different  animals.  The 
most  important  observations  are  those  of  Miescher,  on  the  milt  of  salmon. 
His  analysis  demonstrated  the  presence  of  lecithin,  fat,  and  cholesterin, — 
also  component  parts  of  the  ovum.  Besides  these,  after  the  heads  of  the 
spermatozoa  have  been  formed,  Miescher  detected  the  abundant  presence 
of  a  substance  which  he  called  prota/nin,  which  occurs  in  association  with 
the  }iitcleiii  already  noted  as  present  in  the  yolk.  Albuminoid  material, 
and  products  of  decomposition,  such  as  sarkin  and  guanin,  were  demon- 
strated, according  to  Hensen,  by  Picard. 

Miescher  emphasised  the  interesting  fact,  that  while  the  sperm  is  being 
formed  in  the  Rhine  salmon,  the  animal  is  fasting.  As  no  food  whatever 
is  taken,  and  as  the  muscularity  of  the  fish  is  well  known  to  decrease 
greatly,  Miescher  directly  connected  the  degeneration  of  the  lateral  muscles 
with  the  development  of  the  spermatozoa. 

Zacharias  has  more  recently  made  a  micro-chemical  comparison 
of  the  male  and  female  elements  in  Characerc,  mosses,  ferns,  phanerogams, 
and  amphibians.  Me  finds  that  the  male  cells  are  distinguished  by  their 
small  or  absent  nucleoli,  and  by  their  rich  content  of  nuclein  ;  while  the 
female  elements  exhibited  a  poverty  of  nuclein,  an  abundance  of  albumen, 
and  one  or  more  nucleoli,  more  or  less  large  in  proportion.  The  male  cells 
have,  in  relation  to  their  protoplasm,  a  larger  nuclear  mass  than  the  female 
elements. 

It  is  interesting  to  notice  that  two  investigators  have  recently  pointed 
out,  that  an  analysis  of  two  different  kinds  of  pollen  showed  a  great  analogy 
of  composition  between  these  male  reproductive  cells  and  those  of  the 
salmon  and  ox. 


Il6  THE    EVOLUTION    OF    SEX. 


SUMMARY. 

1.  The  contrast  between  the  elements  is  that  between  the  sexes.  The 
large,  passive,  highly-nourished  anabolic  ovum  ;  the  small,  active,  katabolic 
sperm. 

2.  Hamm's  discovery,  1677  ;  Leeuwenhoek's  interpretation  ;  the  school 
of  animalculists  ;  Kolliker's  demonstration  of  the  cellular  origin  of  the 
sperm,  1841. 

3.  Structure  of  the  sperm, — nuclear  "  head  "  of  chromatin,  protoplasmic 
"tail,"  middle  portion.  The  sperm  in  reality  comparable  to  a  monad  or 
flagellate  infusorian,  only  with  less  cell-substance.  Its  occasional  degra- 
dation into  the  amoeboid  phase. 

4.  Physiology  of  the  sperm  ;  its  locomotor  energy  at  a  maximum,  but 
yet  great  power  of  endurance,  like  a  monad  or  bacillus. 

5.  Origin  of  sperms  from  the  division  of  a  niother-sperm-cell 
homologous  with  the  ovum.  The  different  modes  of  "spermatogenesis" 
may  be  collated  with  the  different  modes  of  ovum-segmentation. 

6.  The  occurrence  in  sperm-development  of  phenomena  comparable 
both  structurally  and  functionally  with  polar-globule  formation. 

7.  Chemistry  of  the  sperm  ;  resemblance  between  pollen  and  sperma- 
tozoa. 


LITERATURE. 

Geddes,  p.,  and  Thomson,  J.  A. — History  and  Theory  of  Spermato- 
genesis. Proc.  Roy.  Soc.  Edin.,  1886,  pyi.  803-823,  i  pi.  See  also 
Zoological  Record  from  1886. 


CHAPTER    X. 

Theory  of  Sex — its  Nature  and  Origin. 

Having  got  so  far  in  our  analysis,  and  before  passing  to  the 
study  of  the  processes  of  reproduction,  we  must  add  up  the 
results  in  a  general  theory  of  the  nature  and  origin  of  sex. 
After  this  has  been  done,  we  shall  be  in  a  better  position  to 
deal,  in  Book  HI.,  with  fertilisation,  parthenogenesis,  and  the 
like.  The  number  of  speculations  as  to  the  nature  of  sex  has 
been  well-nigh  doubled  since  Drelincourt,  in  the  last  century, 
brought  together  two  hundred  and  sixty-two  "  groundless 
hypotheses,''  and  since  Blumenbach  quaintly  remarked  that 
nothing  was  more  certain  than  that  Drelincourt's  own  theory 
formed  the  two  hundred  and  sixty-third.  Subsequent  in- 
vestigators have,  of  course,  long  ago  added  Blumen bach's 
"  Bildungstrieb "  to  the  list ;  nor  is  it  claimed  that  the 
generalisation  we  have  in  our  turn  offered  has  yet  received 
"final  form,"  if  that  phrase  indeed  be  ever  permissible  in  an 
evolving  science,  except  when  applied  to  what  is  altogether 
extinct.  This  much,  however,  is  distinctly  maintained,  that 
future  developments  of  the  theory  of  sex  can  only  differ  in 
degree,  not  in  kind,  from  that  here  suggested,  inasmuch  as  the 
present  theory  is,  for  the  first  time,  an  expression  of  the  facts  in 
terms  which  are  agreed  to  be  fundamental  in  biology,  those  of 
the  anabolism  and  katabolism  of  protoj)lasra. 

§  I.  Suggested  Theories. — According  to  Rolph, — a  fresh  and 
ingenious  thinker,  removed  before  attaining  his  mature  strength, 
— "  the  less  nutritive,  and  therefore  smaller,  hungrier,  and  more 
mobile  organism  [cells,  he  is  speaking  of]  we  call  the  male ; 
the  more  nutritive,  and  usually  more  quiescent  organism  is 
the  female."  He  goes  on  vividly  to  suggest  why  "the  small 
starving  male  cell  seeks  out  the  large  well-nourished  female  cell 
for  the  purposes  of  conjugation,  to  which  the  latter,  the  larger 
and  better  nourished  it  is,  has  on  its  own  motive  less 
inclination." 


Il8  THE    EVOLUTION    OF    SEX. 

Mi  not,  in  his  "  theory  of  gehoblasts,"  or  sexual  elements, 
ventures  little  further  than  regarding  male  and  female  as 
derivatives  of  primitive  hermaphroditism  in  two  opposite 
directions.  "  As  evolution  continued,  hermaphroditism  was 
replaced  by  a  new  differentiation,  in  consequence  of  which  the 
individuals  of  a  species  w-ere — some  capable  of  producing  ova 
only,  others  of  producing  spermatozoa  only.  Individuals  of 
the  former  kind  we  call  females,  of  the  latter  males,  and  they 
are  said  to  have  sex."  "  At  present  all  we  can  say  is,  we  do 
not  know  w^hy  or  how  sexual  individuals  are  produced."  In 
regard  to  the  sex  elements,  we  have  already  noticed  his  opinion 
that  they  are  at  first  "  hermaphroditic  or  asexual,"  and  that 
both  differentiate  by  the  extrusion  or  separation  of  the  con- 
tradictory elements,  the  ovum  getting  rid  of  male  polar  globules, 
the  sperms  leaving  behind  a  female  mother-cell-remnant. 

Brooks  has  emphasised  rather  a  different  aspect  of  the 
question.  "A  division  of  physiological  labour  has  arisen 
during  the  evolution  of  life,  the  functions  of  the  reproductive 
elements  have  become  specialised  in  different  directions." 
"  The  male  cell  became  adapted  for  storing  up  gemmules,  and, 
at  the  same  time,  gradually  lost  its  unnecessary  and  useless 
power  to  transmit  hereditary  characteristics."  "The  males  are, 
as  a  rule,  more  variable  than  the  females  ;  the  male  leads,  and 
the  female  follows,  in  the  evolution  of  new  races."  Brooks 
does  not  exactly  attack  the  problem  of  the  nature  and  origin  of 
sex,  but  his  emphasis  on  the  greater  variability  of  males  is  of 
much  importance. 

These  three  positions  must  be  taken  as  representative ; 
others,  which  appeal  to  superiorities,  polarities,  and  like  mys- 
teries, can  hardly  claim  scientific  standing,  and  have  been 
already  sufficiently  referred  to  at  p.  33.  To  those  which  in- 
terpret the  sexes  in  terms  of  the  advantages  of  sexual  repro- 
duction, and  to  those  which  deal  almost  exclusively  with  the 
problem  of  fertilisation,  w^e  shall  afterwards  return.  The  truth 
in  fact  is,  that  it  is  difficult  to  find  any  answer  at  once  serious 
and  direct  to  the  question  of  the  fundamental  difference  between 
male  and  female. 

5^2.  Natiwe  of  Sex  as  seen  iti  the  Sex-Elements — The  Cell 
Cycle. — As  ova  and  sperms  are  the  characteristic  products  of 
female  and  male  organisms,  it  is  reasonable  that  an  interpretation 
of  sex  should  start  at  this  level.  Here,  assuredly,  the  difference 
between  male  and  female  has  its  fundamental  and  most  con- 


THEORY    OF    SEX ITS    NATURE    AND    ORIGIN. 


119 


centrated  expression.  For  the  bodies,  after  all,  as  Weismann  has 
so  clearly  emphasised,  are  but  appendages  to  this  immortal  chain 
of  sex-cells. 

We  have  already  pointed  out  that  the  sex- cells  are  more  or 
less  on  a  level  with  the  Protozoa,  If  we  only  knew,  they  pro- 
bably differ  widely  from  them  in  those  intricacies  of  nuclear 
structure  of  which  we  only  see  the  surface  ;  yet  as  single  cells  the 
sex-cells  are  comparable  with  the  Protozoa.  For  the  moment, 
let  us  study  those  simplest  organisms.  Even  a  student,  shown 
an  extended  series  of  unicellular  forms,  amcebce,  foraminifers, 


The  divergence  of  male  and  female  cells  from 
primitive  amoeboid  indifference. 

sun -animalcules,  infusorians,  gregarines,  and  some  of  the 
simplest  algDe  as  well,  might  gradually  begin  to  group  these 
in  his  mind  under  three  divisions.  First  there  are  highly  active 
cells, ^ — infusorians  of  all  sorts ;  at  the  opposite  extreme  there 
are  quiescent  forms,  in  which  the  life  seems  to  sleep,  and  loco- 
motion is  almost  absent, — the  gregarines,  and  some  unicellular 
algae  ;  and  between  these  there  are  forms  which  in  a  via  media 
have  effected  a  sort  of  compromise  between  activity  and  pas- 
sivity, which  are  without  the  cilia  of  the  one  or  the  self-contained 
stagnancy  of  the  other,  but  possess  outflow^ings  of  their  living 
substance, — the  familiar  amoeboid  processes.  He  would  thus 
reach,  almost  by  inspection,  a  rough  and  ready  classification 
of  the  Protozoa,  into  active,  passive,  and  amoeboid  cells, — a 


I20 


THE    EVOLUTION    OF    SEX. 


classification  however  which,  under  varying  titles,  is  more  or  less 
distinctly  recognised  by  all  the  authorities  on  the  subject. 

But  if  he  went  further  than  casual  inspection,  and  studied 
the  life-history  of  some  of  the  very  simplest  forms,  such  as  some 
of  the  primitive  moulds  or  Myxomycetes,  and  followed  Hceckel's 


The  encj'sted  Pyotoinyxa,  and  its  division  into  numerous  individuals  within  the  cyst. 

— From  Haickel. 

account  of  the  life  cycle  in  Protomyxa^  he  would  gain  new  light 
on  his  classification.  For  in  these  life-histories  he  would  find 
the  cells  now  encysted,  now  active  lashed  spores,  and  again 
sinking  down  into  the  compromise  of  equilibrium  effected  by 


■V  (  v/ 


The  cyst  o'i Protoviyxa  bursting,  the  flagellate  young  stages  becoming  at  once  amoeboid, 
CT^ntually  to  unite  in  a  composite  amoeboid  mass,  or  "  plasmodium." — After  Haeckel. 


THEORY    OF    SEX — ITS    NATURE    AND    ORIGIN. 


121 


amoeba.  He  is  now  in  a  position  to  recognise  that  the  chapters 
in  the  hfe-history  of  the  simplest  forms  are,  as  it  were,  prophecies 
of  each  of  his  three  groups.  Before  final  differentiation  has 
taken  place,  the  organisms  pass  through  a  cycle  of  phases,  one 
of  which  is  accented  by  each  of  the  different  groups  of  the 
Protozoa.  Thus  an  infusorian  has  its  encysted  chapter,  a 
gregarine  its  amoeboid  stage,  and  a  rhizopod  may  begin  as  a 
mobile  ciliated  spore ;  for  each  group,  while  accenting  one 
phase  of  the  cycle,  retains  embryonic  reminiscences  of  the 
others. 


Diagram  of  the  Cell-cycle, — of  encysted,  ciliated,  and  amtuboid 
phases.  I.,  II.,  III.,  in  Protozoa;  IV.,  ovum  and  sperm  of 
fern  prothallus ;  V.,  encysted,  ciliated,  and  anujeboid 
animal  cells;  VI.,  ciliated  animal  cell  pathologically 
becoming  amccboid  ;  VII.,  sperm  and  amoeboid  sperm; 
VIII.,  ama-boid  and  encj-sted  ovum. — From  Geddes. 

A  conviction  that  the  triple  division  really  meant  much 
would  grow  in  our  student's  mind,  if  he  passed  from  the 
Protozoa  to  the  cells  which  compose  higher  animals.  There  he 
would  find  active  ciliated  cells  in  most  of  the  classes,  from  the 
■Siated  chambers  which  lash  the  water  into  a  sponge,  to  the 
cells  lining  the  air  passages  in  man ;  passive  encysted  cells 
would  be  illustrated  in  some  forms  of  connective,  fatty,  and 


122  THE    EVOLUTION    OF    SEX. 

skeletal  tissue ;  while  the  white  blood  corpuscles  would  be  at 
once  recognised  as  amoebae.  Extended  observation  here  also 
would  show  him  the  cells  passing  from  one  phase  to  another. 
His  rough  classification  of  the  Protozoa  would  be  verified  in  the 
histology  of  higher  animals,  and  would  reappear  in  the  study  of 
their  diseases.  He  would  be  thus  at  length  in  a  position  to 
say,  that  however  these  three  phases  were  brought  about,  the 
forms  characteristic  of  them  were  of  such  wide  occurrence 
through  nature  as  to  justify  his  restatement  of  the  familiar  cell 
theory  in  terms  of  a  larger  conception,  that  of  the  cell-cycle ; 
that  is  to  say,  from  the  conception  of  the  cell  as  a  unit  mass 
of  living  protoplasm,  amoeboid,  encysted,  or  ciliated,  as  the 
case  might  be,  he  would  come  to  regard  these  forms  as  the 
predominant  phases  of  a  cycle, — primeval,  certainly,  in  the 
history  of  the  organic  world,  and  largely  so  even  in  the 
individual  cell. 

All  this  time,  however,  our  student  has  remained  a  mor- 
phologist,  his  use  of  terms,  like  active  and  passive,  simply 
expressing  change  of  place.  Not  on  this  path  of  structural 
observation  alone  is  it  possible  to  understand  what  the  forms 
and  i)hases  of  cells  really  mean.  A  final  corroboration  of  the 
cell-cycle,  and  at  the  same  time  a  rationale  of  it,  is  obtainable 
only  on  physiological  lines,  when  we  begin  to  inquire  into  the 
protoplasmic  processes  which  lie  behind  any  change  in  the 
form  and  habit  of  a  cell.  We  have  already  spoken  of  the 
modern  physiologist's  conception  of  living  matter,  or  proto- 
plasm, as  an  exceedingly  complex  and  unstable  substance  or 
mixture  of  substances,  undergoing  continual  chemical  change 
or  metabolism.  On  the  one  hand,  it  is  being  continually 
reconstructed  by  an  income  of  nutritive  material,  which,  at 
first  more  or  less  simple,  is  worked  up  by  a  series  of  chemical 
changes  till  it  reaches  the  climax  of  complexity  and  instability. 
These  upbuilding,  constructive,  synthetic  processes  are  summed 
up  in  the  phrase  anabolism.  But,  on  the  other  hand,  the  proto- 
plasm is  continually,  as  it  "lives,"  breaking  down  into  more  and 
more  stable  compounds,  and  finally  into  waste  products.  There 
is  a  disruptive,  descending  series  of  chemical  changes  known  as 
katabolism.  Both  constructive  and  disruptive  changes  occur  in 
manifold  series.  The  same  summit  (see  p.  89)  may  be  gained  or 
left  by  many  different  paths,  but  at  the  same  time  there  is,  as  it 
were,  a  distinct  watershed, — any  change  in  the  cell  must  tend  to 
throw  the  preponderance  towards  one  side  or  the  other.     In  a 


THEORY    OF    SEX — ITS    NATURE    AND    ORIGIN.  I  23 

certain  sense  too  the  processes  of  income  and  expenditure 
must  balance,  but  only  to  the  usual  extent,  that  expenditure 
must  not  altogether  outrun  income,  else  the  cell's  capital  of 
living  matter  will  be  lost, — a  fate  which  is  often  not  successfully 
avoided.  The  disruptive,  or  katabolic,  or  energy- expending 
set  of  changes,  may  be  obviously  greater  in  one  cell  than  in 
another,  in  proportion  to  the  constructive  or  anabolic  processes. 
Then,  we  may  shortly  say  that  the  one  cell  is  more  katabolic 
than  the  other,  or  vice  versa  on  the  opposite  supposition.  Just 
as  our  expenditure  and  income  should  balance  at  the  year's 
end,  but  may  vastly  outstrip  each  other  at  particular  times,  so 
it  is  with  the  cell  of  the  body.  Income  too  may  continuously 
preponderate,  and  we  increase  in  wealth,  or  similarly,  in  weight, 
or  in  anabolism.  Conversely,  expenditure  may  predominate, 
but  business  may  be  prosecuted  at  a  loss  ;  and  similarly,  we 
may  live  on  for  a  while  with  loss  of  weight,  or  in  katabolism. 
This  losing  game  of  life  is  what  we  call  a  katabolic  habit,  tend- 
ency, or  diathesis;  the  converse  gaining  one  being,  of  course,  the 
anabolic  habit,  temperament,  tendency,  or  diathesis.  The  words 
anabolic  and  katabolic  are,  of  course,  new,  unfamiliar,  and  un- 
deniably ugly.  Habit  and  temperament  have  very  vague  associa- 
tions, and  tendency  sounds  metaphysical;  diathesis,  again,  seems 
no  better  than  the  medical  equivalent  of  this.  These  things  the 
reader  must  naturally  feel;  yet  the  medical  man  is  now-a-days 
quite  scientific  and  definite  in  speaking  of  gouty  or  neurotic 
diathesis,  of  bilious  habit,  strumous  tendency,  or  the  like.  The 
metaphysical  vagueness  is  no  longer  chargeable  to  him ;  still  less, 
we  trust,  to  us. 

We  are  now  in  a  position  profitably  to  return  to  the  Pro- 
tozoa, to  the  phases  of  cell-life,  and  to  the  sex-elements.  After 
what  we  have  just  said,  it  is  evident  that  there  are  but  three 
main  physiological  possibilities, — preponderant  anabolism,  or 
predominant  katabolism,  or  an  approximate  (/.<?.,  oscillating) 
equilibrium  between  these  tendencies.  A  growing  surplus  of 
income,  a  lavish  expenditure  of  energy,  or  a  compromise  in 
which  the  cell  lives  neither  far  below  nor  quite  up  to  its 
income.  Great  passivity,  great  activity,  or  a  safe  average 
between  these;  conservative  accumulation,  spendthrift  liberal- 
ism, and  a  compromise  between  these.  In  many  different 
ways,  more  or  less  metaphorical,  may  we  express  the  plain  and 
indubitable  facts  of  anabolism  and  katabolism  within  the  living 
matter.     The  student  may  think  of  the  processes,  with  some 


124  THE    EVOLUJION    OF    SEX. 

degree  of  accuracy,  under  the  metaphor  of  an  eddy  in  a  stream, 
or  of  a  ceaseless  fountain,  which,  while  remaining  approximately 
constant,  is  the  expression  of  continual  ascent  and  descent  of 
drops.  The  protoplasm  itself  must  often  be  in  as  ceaseless 
change  as  the  apex  of  the  jet. 

In  active,  motile,  ciliated,  or  flagellate  cells,  whether  they  be 
constant  forms  or  only  temporary  phases,  there  is  predominant 
katabolism, — predominant  when  compared  with  the  life  expen- 
diture of  a  passive,  quiescent,  enclosed,  or  encysted  cell.  In 
amoeboid  organisms  these  extremes  are  avoided  ;  there  is  cer- 
tainly great  amplitude  of  variation  still,  but  neither  anabolism 
nor  katabolism  gains  the  ascendant  in  any  marked  degree. 

Suppose,  then,  in  such  an  amoeboid  cell,  a  continued 
surplus  of  anabolism  over  katabolism,  the  result  is  necessarily 
a  growth  in  size,  a  reduction  of  kinetic  energy  and  movement, 
an  increase  in  potential  energy  and  reserve  food-material. 
Irregularities  will  tend  to  disappear,  surface-tension  too  may 
aid,  and  the  cell  acquires  a  spheroidal  form.  The  result  — 
surely  intelligible  enough — is  a  large  and  quiescent  ovum. 

It  will  be  remembered  that  young  ova  are  very  frequently 
amoeboid ;  that  with  a  copious  nutrition  this  disappears  in 
varying  degrees  of  encystment ;  that  ensheathing  envelopes 
arising  from  the  ovum,  sw^eated  off  like  cysts  round  Protozoa, 
are  exceedingly  common  ;  and  that  ova  are  the  largest  of  all 
animal  cells. 

Starting  once  more  from  an  amoeboid  cell,  if  katabolism 
comes  to  be  more  and  more  predominant,  the  increasing  libera- 
tion of  kinetic  energy  thus  implied  must  find  its  outward 
expression  in  increased  activity  of  movement  and  in  diminished 
size  ;  the  more  active  cell  l)ecomes  modified  in  form,  in  adapta- 
tion to  passage  through  its  fluid  environment,  and  the  natural 
result  is  a  flagellate  sperm. 

In  short,  then,  the  respective  mori)hological  characters  of  the 
sex-cells,  female  and  male,  find  the  same  physiological  rationale  as 
do  the  large  passive  encysted  and  smaller  active  ciliated  phases 
of  the  cell-cycle  in  general,  and  are  alike  the  outcome  and 
expression  of  predominant  anabolism  and  katabolism  respec- 
tively. Here  again  we  reach  the  same  formula  as  before ;  or, 
more  cumbrously  in  words — the  functions  are  either  self-main- 
taining or  species  maintaining,  individual  or  reproductive ;  the 
former  are  divided  into  anabolic  and  katabolic,  the  latter  into 
male  and  female.    But  the  second  set  of  products  and  processes, 


THEORY    OF    SEX  —  ITS    NATURE    AND    ORIGIN. 


125 


SO  far  from  being  unrelated  to  the  other  as  is  commonly 
supposed,  are  in  complete  parallelism.  Femaleness  is  anabolic 
preponderance  in  reproduction,  hence  the  ovum  has  necessarily 
the  general  character  which  this  "diathesis"  produces  in  non- 
reproductive  cells ;  and,  similarly,  katabolic  preponderance 
stamps  its  character  of  active  energy  upon  spermatozoon  as 
naturally  as  upon  the  ciliated  cell  or  the  monad. 


Diagram  showing  the  divergence  of  ovum  and  spermatozoon 
from  a  undifferentiated  amoeboid  type  of  cell. 

Rolph's  characterisation  of  tlie  male  cells  as  hungry  and  starving 
(katabolic),  has  been  experimentally  confirmed  by  their  powerful  attrac- 
tion to  highly  nutritive  fluids,  and  is  every  day  illustrated  in  their  persistent 
attraction  to  the  ova.  Platner  has  suggested,  in  the  intimately  hermaphrodite 
gland  of  the  snail,  that  the  external  cells  which  form  the  ova  are  better 
nourished  than  the  central  cells  which  divide  into  sperms.  Just  as  an 
infusorian  in  dearth  of  food  is  known  in  some  cases  to  divide  into  many 
small  individuals,  so  the  mother-sperm-cell  is  perhaps  the  seat  of  similar 
katabolic  necessities.  The  long  persistence  of  vitality  seems  at  first  sight  a 
difficulty,  if  the  sperms  are  highly  katabolic  cells.  It  must  be  noticed, 
however,  {a)  That  there  is  often  only  retention,  not  continuance  of  activity, 
e.^i^. ,  when  the  sperms  lie  closely  packed  in  the  special  storing  reservoirs  ; 
{/>)  That  the  secretions  of  the  female  ducts  probably  afford  some  nutriment 
to  the  sperms,  which  expose  an  exceptionally  large  surface  in  proportion  to 
their  mass  ;  and  (c)  That   to  a  certain  extent  we  may  think  of  them  as 


126  THE    EVOLUTION    OF    SEX. 

protoplasmic  explosives,  which  may  remain  long  inert,  but  on  the  presence 
of  the  required  stimulus  are  able  to  start  again  into  extraordinary  activity. 

§  3.  The  Problem  of  the  Origin  of  Sex. — We  must  now 
return  once  more  to  the  standpoint  of  the  empirical  naturahst, 
and  set  out  towards  the  interpretation  of  sex  from  a  different 
side,  that  of  its  origin. 

It  has  often  been  raised  as  a  reproach  against  the  now 
fortunately  dominant  school  of  evolutionist  naturalists,  that 
they  could  give  no  account  of  the  origin  of  sex.  Some  people, 
like  children,  wish  everything  at  once.  Yet  it  must  be  admitted 
that  there  has  been  a  lack  of  any  sure  and  certain  voice  on  this 
question.  Apart  from  the  simple  fact  that  evolutionist  biology 
is  still  young,  there  are  three  reasons  for  the  comparative 
silence  in  regard  to  the  origin  of  sex. 

(i.)  The  first  of  these  is  the  still  curiously  prevalent  opinion, 
that  when  you  have  explained  the  utility  or  advantage  of  a 
fact,  you  have  accounted  for  the  fact, — an  opinion  which  the 
theory  of  natural  selection  has  done  more  to  foster  than  to 
rebuff.  Darwin  was,  indeed,  himself  characteristically  silent  in 
regard  to  the  origin  of  sex,  as  well  as  of  many  other  "  big  lifts  " 
in  the  organic  series.  Many,  however,  have  from  time  to  time 
pointed  out  that  the  existence  of  male  and  female  was  a  good 
thing.  Thus  Weismann  finds  in  sexual  reproduction  the  chief, 
if  not  the  sole  source  of  progressive  change.  Be  that  as  it 
may  at  present,  it  is  evident  that  a  certain  pre-occupation 
with  the  ulterior  benefits  of  the  existence  of  male  and  female, 
may  soniewhat  obscure  the  question  of  how  male  and  female 
have  in  reality  come  to  be. 

(2.)  A  second  reason  for  the  comparative  silence,  may  be 
found  in  the  fact  that  the  problem  remains  insoluble  until  it  is 
analysed  into  its  component  problems.  The  question  of  the 
origin  of  sex  to  a  mind  unprepared  for  the  consideration  of 
such  a  problem,  suggests  quite  a  number  of  difiiculties, — What 
is  the  import  and  origin  of  sexual  reproduction  (the  setting 
apart  of  special  cells)  ?  what  is  the  meaning  and  beginning  of 
fertilisation  (the  interdependence  and  union  of  sex-cells)  ?  what 
is  the  reason  of  the  individual,  male  or  female,  sex  in  any 
one  case  (the  determination  of  sex)  ?  and  lastly,  what  is  the 
nature  and  origin  of  the  difference  between  male  and  female  ? — 
the  question  at  present  under  discussion.  For  purposes  of 
analysis,  those  questions  must  be  kept  distinct,  though  in  the 
final  synthesis  they  are  all  answerable  in  a  sentence. 


THEORY    OF    SEX — ITS    NATURE    AND    ORIGIN.  127 

(3.)  A  third  reason  why  the  problem  of  the  origin  of  male 
and  female  has  been  so  much  shirked,  why  naturalists  have 
beaten  so  much  about  the  bush  in  seeking  to  solve  it,  is  that  in 
ordinary  life,  for  various  reasons,  mainly  false,  it  is  customary 
to  mark  off  the  reproductive  and  sexual  functions  as  facts 
altogether  per  se.  Modesty  defeats  itself  in  pruriency,  and 
good  taste  runs  to  the  extreme  of  putting  a  premium  upon 
ignorance.  Now  this  reflects  itself  in  biology.  Reproduction 
and  sex  have  been  fenced  off  as  facts  by  themselves ;  they  have 
been  disassociated  from  the  general  physiology  of  the  individual 
and  the  species.  Hence  the  origin  of  sex  has  been  involved 
in  special  mystery  and  difficulty,  because  it  has  not  been 
recognised  that  the  variation  which  first  gave  rise  to  the 
difference  between  male  and  female,  must  have  been  a  varia- 
tion only  accenting  in  degree  what  might  be  traced  universally. 

§  4.  N^atiwe  of  Sex  as  see?i  in  its  Origin  among  Plants. 
— In  tracing  the  origin  of  sex,  w^e  would  wish  to  guard 
against  any  impression  of  having  consciously  or  unconsciously 
arranged  our  facts  in  the  light  of  the  theory  we  hold.  Hence 
we  prefer  to  follow^  some  accessible  account,  taken  essentially 
from  the  morphological  j)oint  of  view.  We  shall  follow  Prof. 
Vines  in  his  article  Reproduction —  Vegetable,  in  the  Eucyclopcedia 
Britannica,  at  each  stage,  however,  endeavouring  to  interpret 
the  facts,  physiologically,  in  the  light  of  protoplasmic  processes. 

(i.)  The  simple  alga,  Protococciis — which,  in  the  widest  sense  of  that 
term,  every  one  knows  in  some  form  or  other,  on  tree-stems,  in  pools, 
wells,  and  the  like — reproduces  itself  in  a  simple  fashion.  The  cell  divides 
into  a  number  of  equal  units  or  spores  ;  these  are  set  free,  are  mobile  for  a 
while,  eventually  come  to  rest,  and  develop  to  the  normal  size.  A  hint, 
however,  of  the  beginning  of  a  difference  is  seen  when  the  cell  occasionally 
divides  into  a  larger  number  of  smaller  spores.  These,  however,  show  no 
difference  in  history.  They  settle  down,  and  develop  just  like  their  more 
richly-dowered  neighbours.  We  find  here  the  occurrence  of  units  of  smaller 
size,  that  is  to  say,  less  predominantly  anabolic,  but  still  these  are  able  to 
develop  independently. 

(2.)  In  a  higher  alga,  Ulothrix — one  of  the  series  known  as  Confervoe — 
both  large  and  small  reproductive  cells  are  developed.  The  large  ones 
develop  always  of  themselves,  and  so  may  the  smaller  forms.  But  the 
smaller  forms  may  also  unite  in  pairs,  and  then  start  a  new  plant  from  the 
double  capital  thus  attained.  When  one  of  the  smaller  cells  develops  by 
itself,  the  result,  in  some  cases  at  least,  is  a  weakly  plant.  They  have  what 
Prof  Vines  calls  an  "imperfect  sexuality,"  for  while  they  are  in  part 
dependent  upon  union  with  other  cells,  they  are  not  wholly  so.  They  are 
anabolic  enough,  we  may  say,  sometimes  to  develop  independently,  but 
often  they  are  individually  too  katabolic  for  anything  but  weak  independent 
development.     In  uniting,  however,  in  mutual  nutrition,  they  are  strong. 


128  THE    EVOLUTION    OF    SEX. 

The  student  will  already  see  the  relative  femaleness  of  the  large  units,  the 
maleness  of  their  smaller  neighbours. 

(3.)  A  third  stage  is  reached  in  another  alga,  Ectocarpiis,  which  is 
peculiarly  instructive.  This  may  separate  off  large  cells  which  develop  by 
themselves  like  parthenogenetic  ova.  From  other  parts  of  the  plant 
smaller  units  are  liberated,  which  generally,  though  not  yet  invariably, 
unite  with  one  another  before  developing.  But  between  these  smaller  units 
a  most  important  physiological  difference  has  been  observed  by  Berthold. 
Some  soon  come  to  rest  and  settle  dnwn,  and  with  these  their  more 
energetic  neighbours  by-and-by  unite.  We  have  here  a  very  distinct 
beginning  of  the  distinction  between  male  and  female  elements.  The 
comparatively  sluggish,  more  nutritive,  preponderatingly  anabolic  cells, 
which  soon  settle  down — are  female ;  the  more  mobile,  finally  more 
exhausted  and  emphatically  katabolic  cells — are  male.  As  Vines  says, 
"  the  one  is  passive,  the  other  active  ;  the  former  is  to  be  regarded  as  the 
female,  and  the  latter  as  the  male  reproductive  cell." 

(4.)  Further,  in  another  alga,  Ciitleria,  the  differentiation  may  be 
traced.  Two  kinds  of  units  result,  which  must  unite  with  one  another  if 
development  is  to  take  place,  but  these  units  arise  from  perfectly  distinct 
sources  in  the  parent  plant.  The  larger  less  mobile  cells,  which  soon  come 
to  rest,  are  fertilised  by  the  smaller  more  active  units.  The  more  anabolic 
or  female  cells  are  fertilised  by  the  more  katabolic  or  male  cells,  which 
have  now  gone  too  far  for  the  possibility  of  independent  development. 

(5.)  To  complete  the  series,  we  may  simply  mention  such  a  case  as  that 
to  which  we  shall  presently  return,  —  those  forms  of  Volvox,  where  an 
entire  colony  of  cells  produces  either  female  or  male  elements,  thus  repre- 
senting the  beginning  of  an  entirely  unisexual  many-celled  organism. 

While  the  above  cases  also  involve  the  problem  of  the 
origin  of  fertilisation,  which  is  left  over  for  the  present,  they 
confirm  most  clearly  our  general  conclusion  that  preponderant 
katabolism  or  anabolism  are  the  ruling  characteristics  of  male 
or  female  respectively. 

i:^.  5.  Nature  of  Sex  as  seen  in  Origin  among  Animals. — 
Among  the  Protozoa  also,  we  can  trace  the  beginnings  of  the 
same  "dimorphism"  between  male  and  female.  A  union 
between  similar  cells  is  of  course  frequent,  but  that  is  not  at 
present  to  the  point.  What  we  refer  to,  are  the  numerous  cases, 
especially  among  flagellate  and  vorticella-like  infusorians, 
where  the  two  individuals  which  unite  are  quite  unlike  one 
another  both  in  form  and  history.  "  There  can  be  no  doubt," 
Hatchett  Jackson  remarks,  "  that  the  process  is  essentially  a 
sexual  one ;  when  the  individuals  are  invariably  different,  there 
is  no  reason  why  the  terms  male  and  female  should  not  be 
applied  to  them."  In  some  cases  we  find  as  before  that  a  small 
active  katabolic  unit  combines  with  a  larger,  more  passive, 
and  anabolic  individual. 


THEORY    OF    SEX — ITS    NATURE    AND    ORIGIN. 


129 


In  the  bell-animalcule,  which  grows  so  commonly  on  the 
water-plants  of  our  ponds,  a  minute  free-swimming  unit,  formed 
as  one  of  the  results  of  repeated  division,  unites  with  a  stalked 
individual  of  the  normal  size.  In  the  related  Epistylis,  Engel- 
mann  has  described  how  an  individual  divides  first  of  all  into 
two  cells.     One  of  these  remains  as  such  (like  an  ovum),  the 


Vorticella,  the  Bell-animalcule,— a,  the  normal  individual ; 
b,  its  division  into  two  ;  c,  the  division  accomplished  ; 
d,  the  further  division  of  one  of  the  halves  into  eight 
small  (male)  units  ;  e,  a  minute  individual  uniting  with 
one  of  normal  size. 

Other  repeatedly  divides  (like  a  mother-sperm-cell)  into  numerous 
minute  units.  One  of  these  subsequently  unites  with  the 
undivided  cell,  and  Engelmann  does  not  hesitate  to  call  the 
different  elements  male  and  female.  In  some  radiolarians 
{e.g.,  Collozoiim),  dimorphic  spores — large  and  small — have  been 
described,  although  their  history  has  not  yet  been  fully  traced. 
Even  in  Foraminifera,  as  Schlumberger,  De  la  Harpe,  and  H. 
B.  Brady  have  shown,  a  marked  dimorphism  may  occur ;  and 
here  again  the  distinction  seems  to  lie  between  preponderant 
anabolism  and  katabolism. 

As  another  illustration,  it  will  be  instructive  to  select  the 
case  of  volvox.  In  this  colonial  organism,  which  is  best  re- 
garded as  a  multicellular  protist,  the  component  cells  are  at  first 
all  alike.  They  are  united  by  protoplasmic  bridges,  and  simply 
form  a  vegetative  colony.  In  favourable  environmental  con- 
ditions this  state  of  affairs  may  persist,  or  be  interrupted  only 
by  parthenogenetic  multiplication.  When  nutrition  is  checked, 
however,  sexual  reproduction  makes  its  appearance,  and  that  in 
a  manner  which  illustrates  most  instructively  the  differentiation 
of  the  two  sets  of  elements.  Some  of  the  cells  are  seen 
differentiating  at  the  expense  of  others,  accumulating  capital 

I 


130  THE    EVOLUTION    OF    SEX. 

from    their    neighbours ;    and   if  their  area  of  exploitation  be 
sufficiently  large,   emphatically  anabolic   cells    or    ova   result ; 


I  oh'ox  glohator,  a  colonial  Alga  or  Infusorian,  showing  the 
ordinary  cells  (c)  that  make  up  the  colony  (or  body),  and 
the  special  reproductive  cells  (a,  d),  both  male  and 
female. — After  Cohn. 

while  if  their  area  is  reduced  by  the  presence  of  numerous 
competitors  struggling  to  become  ova,  the  result  is  the  forma- 
tion of  smaller,  less  anabolic  cells,  which  become  ultimately 
viale,  segment  into  antherozoids,  meantime  losing  their  vegeta- 
tive greenness  and  becoming  yellow.  In  some  species,  distinct 
colonies  may,  in  the  same  way,  become  predominantly  anabolic 
or  katabolic,  and  be  distinguishable  as  completely  female  or 
male  colonies.  Thus,  again,  we  reach  the  conclusion,  of  a 
predominant  anabolism  effecting  the  differentiation  of  female 
elements,  and  of  katabolism  as  characteristic  of  the  male. 

^5  6.  Corroborative  IlIiisfraiio?is. — If  the  anabolic  and  kata- 
bolic contrast,  so  plainly  seen  in  the  sex-elements,  be  the  funda- 
mental one,  we  must  expect  to  find  it  saturating  through  the 
entire  organism.  We  have  already  drawn  attention  to  the 
occurrence  of  yolk  glands  in  association  with  ovaries.  Or 
again,  in  the  cells  of  a  developing  anther  an  enormous  number 


THEORY    OF    SFX — ITS    NATURE    AND    ORIGIN. 


131 


of  crystals  may  be  often  observed  to  occur.  Crystals  are,  how- 
ever, usually  regarded  as  accumulations  of  waste  products, 
and  these  anther  crystals  are,  in  fact,  comparable  to  urinary 
deposits.  Such  accumulations  do  not,  however,  occur,  at  least 
to  any  similar  extent,  in  the  embryo-sac  or  in  the  female  organs, 
in  spite  of  the  homology  in  male  and  female  development. 
They  occur  as  results  of  katabolism,  where  we  would  naturally 
expect  them — in  the  tissue,  of  male  organs. 


A  Stonewort  {Charafragilis),  showing  in  two  stages,  adult  and 
embryonic,  the  female  organ  {b),  and  the  male  organ  (a). — 
From  Sachs,  after  Pringsheim. 

In  the  stoneworts  Chara  or  Nitella  there  is,  as  is  well 
known,  an  alternation  between  nodal  and  internodal  cells. 
The  internodal  cells  are  actively  vegetative,  and  go  on 
increasing  in  size;  they  do  not  divide,  and  may  be  jusdy 
regarded  as  emphatically  anabolic.  The  nodal  cells,  on  the 
other  hand,  are  much  smaller,  and  do  divide.  That  is  to  say 
they  are  relatively  more  katabolic. 

A  crucial  test  of  the  present  theory  thus  suggests  itself. 


132  THE    EVOLUTION    OF    SEX. 

Since  the  reproductive  organs  are  simply,  as  every  morphologist 
knows,  shortened  branch-structures,  we  should  predict  that  the 
cell  from  the  segmentation  of  which  the  antheridium  is  derived 
must  correspond  in  position  to  a  nodal  and  katabolic  cell  (/.^., 
be  based  upon  an  internode),  while  the  corresponding  essentially 
female  cell  or  ovum  must  be  internodal  or  apical  in  origin  {i.e., 
based  upon  a  node,  and  this  relatively  more  anabolic).  It  is 
therefore  not  a  little  noteworthy  that  an  examination,  alike  of 
classical  figures  and  fresh  specimens,  will  show  that  this  imper- 
fect homology,  but  perfect  physiological  correspondence,  is 
invariably  the  fact  (see  figure). 

§  7.  Co7iclusion. — In  conclusion,  in  defiance  of  Dr  Minot's 
recent  dictum,  that  "such  speculation  passes  far  beyond  the 
present  possibilities  of  science,"  we  believe  that  the  consideration 
\d)  of  the  characteristics  of  the  sex-elements,  alike  in  history,  as 
Minot  himself  emphasises,  and  in  their  finished  form,  (^)  of 
the  incipient  sex  dimorphism  seen  among  the  simplest  plants 
and  animals,  {c)  of  phenomena,  both  normal  and  pathological, 
in  the  sexual  tissues  and  organs,  [d)  of  the  established  facts 
in  regard  to  the  determination  of  sex  (chap.  4),  and  {e)  of  the 
structural  and  functional,  primary  and  secondary  characteristics 
of  the  sexes  (chap.  2  and  passtjn), — all  lead  to  the  general  con- 
clusion, that  the  female  is  the  outcome  and  expression  of  pre- 
ponderant anabolism,  and  in  contrast  the  male  of  predominant 
katabolism.  Further  corroborations  will  gradually  appear  in 
the  succeeding  sections,  as  we  discuss  fertilisation,  partheno- 
genesis, or  special  facts  like  menstruation  and  lactation.  The 
whole  thesis  may  be  once  more  summed  up  diagrammatically. 

SUM   OF    FUNCTIONS. 


Nutrit  on.  Reproduction. 


Anabolism.  Katabolism.  Female.  Male. 

In  this  way  we  see,  with  reference  to  the  three  speculations 


THEORY    OF    SEX ITS    NATURE    AND    ORIGIN.  1 33 

outlined  at  the  beginning  of  the  chapter, — (i.)  that  the  penetrat- 
ing insight  of  Rolph,  of  females  as  the  more,  and  males  as  the 
less  nutritive,  is  fully  justified;  (2.)  that  the  view  of  Minot  of  the 
differentiation  of  both  sex-cells  from  a  primitive  hermaphroditism 
becomes  similarly  developed,  and  acquires  greater  definiteness ; 
while  (3.)  the  view  of  Brooks,  which  ascribes  variability  primarily 
to  the  males,  at  least  acquires  considerable  suj)port  from  the  inter- 
pretation of  the  males  as  preponderatingly  katabolic.  For  it  is 
rather  in  connection  with  the  destructive  changes  of  ])rotoplasm 
than  with  the  constructive,  that  variations  might  be  expected  to 
arise. 


134  THE    EVOLUTION    OF    SEX. 


SUiMiMARV 


1.  Suggested  theories  of  the  nature  of  male  and  female  ;  their  number 
and  vagueness.  Three  recent  developments — (a)  Rolph's  penetrating  sug- 
gestion of  more  nutritive  females,  less  nutritive  males  ;  (/')  Minot's  tlieory 
of  the  differentiation  of  both  kinds  of  sex-cells  from  a  primitive  her- 
maphroditism ;  {c)  the  conclusion  of  Brooks,  that  the  males  are  more  vari- 
able, and  alone  transmit  new  variations. 

2.  Nature  of  sex  seen  in  its  essence  in  the  sex-cells.  The  fundamental 
protoplasmic  antithesis  illustrated  in  the  Protozoa,  in  the  cells  of  higher 
animals,  in  life-histories.  The  conception  of  a  cell-cycle.  The  physiolo- 
gical import  of  this, — the  protoplasmic  possibilities,  preponderant  ana- 
bolism,  predominant  katabolism,  and  a  relative  equilibrium.  The  anabolic 
character  of  the  ova.      The  katabolic  character  of  the  sperms. 

3.  The  problem  of  the  origin  of  sex,  so  little  tackled,  because  of  (a)  the 
blinding  influence  of  teleological  or  utilitarian  inquiries,  (/-')  the  number  of 
separate  problems  involved,  {c)  the  isolation  of  sex  and  reproduction  from 
the  general  life  of  the  organism  and  species. 

4.  A  series  from  simple  plants,  showing  the  gradual  appearance  of 
dimorphic  sex-cells,  with  the  physiological  interpretation  thereof.  The 
dimorphism  is  the  result  of  preponderant  katabolism  and  anabolism,  and 
this  is  the  origin  of  male  and  female. 

5.  Illustrations  of  incipient  dimorphism  or  sex  among  the  Protozoa. 
Special  reference  to  the  case  of  volvox. 

6.  Corroborative  illustrations, — anther  cells  and  Chara. 

7.  General  conclusion, — (a)  from  the  sex-cells,  {i)  from  incipient  sex,  (c) 
from  organs  and  tissues,  (c/)  from  the  determination  of  sex,  {e)  from 
the  characters  of  the  sexes, — that-  male  and  female  are  the  results  and 
expressions  of  i:)redominant  katabolism  and  anabolism  respectively,  with 
consequent  confirmation  of  the  speculations  of  Rolph  and  Minot,  and  in 
some  measure  also  of  that  of  Brooks. 


LITERATURE. 

Brooks,  \V.  K.— The  Law  of  Heredity.     Baltimore,  1SS3. 

Geddes,  p.  —  0/>/>.  c/L,  especially  "  Theory  of  Growth,  Reproduction,  Sex, 
and  Heredity,"  Proc.  Roy.  Soc.  Edin.,  18S6  ;  and  Article  "Sex," 
Encyc.  Brit.,  also  "Restatement  of  Cell  Theory,"  Proc.  Roy.  Soc. 
Edin.,  1883-84. 

Minot,    C.    S.--Theorie    der    Genoblasten.       Biolog.    Centralblatt,    II., 

P-  365. 

Rolph,  W.  H. — Biologische  Probleme.     Leipzig.  1S84. 

Sachs,  J. — Text-book  of  Botany,  edit,  by  Vines,  second  edition,  1882  ; 
and  Physiology  of  Plants,  translated  by  Marshall  Ward,  1887. 

Vines,  S.  H. — Physiology  of  Plants,  1886  ;  article  "  Reproduction- 
Vegetable,"  Encyc.  Brit. 

Welsmann,  a.  —  0/>/>.  lit. 


BOOK     III 


M»>< 


PROCESSES    OF    REPRODUCTION, 


CHAPTER  XI. 

Sexual  Reproduction. 

§  I.  Dif event  Alodes  of  Reproduction. — It  is  well  known  that 
a  starfish  deprived  of  an  arm  can  replace  this  by  a  fresh 
growth ;  that  crabs  can  renew  the  great  claws  which  they  have 
lost  in  fighting  ;  and  that,  even  as  high  up  as  the  lizards,  the 
loss  of  a  leg  or  a  tail  can  be  made  good.  In  a  great  variety  of 
cases,  a  kind  of  physiological  forgiveness  is  shown  in  the  repara- 
tion of  even  serious  injuries.  Now  this  "regeneration,"  as  it  is 
called,  is  in  a  certain  degree  a  process  of  reproduction.  By 
continuous  growth  the  cells  of  a  persistent  stump  are  able  to 
reproduce  the  entire  member,  "We  know  too  that  a  sponge,  a 
hydra,  or  a  sea-anemone,  may  be  cut  into  pieces,  with  the  result 
that  each  fragment  grows  into  a  new  organism.  The  same  is 
done  with  many  plants ;  and  though  the  division  is  artificial, 
the  result  shows  how  very  far  from  unique  is  the  process  which 
we  usually  speak  of  as  reproduction.  In  fact,  as  Spencer  and 
Hgeckel  said  long  ago,  reproduction  is  but  more  or  less  discon- 
tinuous growth.  So  again,  we  pass  onwards  insensibly  from 
cases  of  continuous  budding,  as  in  sponge  or  rose-bush,  to 
discontinuous  budding  in  hydra,  zoophyte,  and  tiger-lily,  where 
the  offspring,  vegetatively  produced,  are  sooner  or  later  set  free. 
Similarly  in  the  Protozoa,  an  almost  mechanical  breakage  begins 
the  series.  This  becomes  more  definite,  in  the  production  of 
several  buds  at  once,  or  of  only  one.  Budding  leads  on  to 
deliberate  and  orderly  division,  both  multiple  and  binary : 
while  finally,  in  colonial  forms,  the  liberation  of  special  repro- 
ductive units  may  be  observed. 

We  shall  afterwards  have  to  discuss  the  relations  of  these 
and  other  processes  ;  but  just  as  we  began  the  study  of  sex 
with  the  familiar  contrast  of  male  and  female,  so  we  shall  begin 
our  investigation  of  the  reproductive  processes  with  the  most 
obtrusive  mode,  known  as  sexual  reproduction. 


138  THE    EVOLUTION    OF    SEX. 

^  2.  Facts  Involved  i)i  Sexual  Reproduction. — It  is  necessary, 
at  the  outset,  to  be  quite  clear  as  to  the  concurrence  of  several 
distinct  facts  in  any  ordinary  case  of  sexual  reproduction  among 
many-celled  organisms,  (i.)  There  is,  first  of  all,  the  fact  that 
special  reproductive  cells  are  present  in  more  or  less  marked 
contrast  to  the  ordinary  cells  making  up  the  body.  To  this 
antithesis  we  have  already  given  due  prominence.  (2.)  Then 
there  is  the  further  fact,  that  these  special  reproductive  cells  are 
dimorphic  ;  that  they,  and  the  organisms  which  produce  them, 
are  distinguishable  as  male  and  female.  This  has  been  the 
main  theme  of  the  two  preceding  books.  (3.)  Lastly,  we  have 
to  recognise  that  these  dimorphic  sex-cells  are  mutually 
dependent, — that  if  the  egg-cell  is  to  develop  into  an  organism, 
it  must  first  be  fertilised  by  a  male  element.  On  the  facts  of 
fertilisation,  therefore,  as  observed  in  plants  and  animals,  atten- 
tion must  now  be  concentrated. 

§  3.  Fertilisation  in  Plants.  — "  The  Newly  Discovered 
Secret  of  Nature  in  the  Structure  and  Fertilisation  of  Flowers," 
so  ran  the  title  of  a  work  published  by  Conrad  Sprengel  in 
1793,  embodying  his  pioneer  investigations  on  a  now  familiar 
field.  Though  not  indeed  the  first  to  point  out  the  importance 
of  insects  in  relation  to  fertilisation,- — for  that  honour  appears 
to  belong  to  Kolreuter  (1761), — Sprengel  laid  sure  foundations, 
now  somewhat  hidden  by  the  superstructure  which  Darwin  and 
others  have  built.  To  Sprengel's  eyes,  the  many  ways  in  which 
the  nectar  is  protected  from  rain  seemed  full  of  "  intention." 
He  recognised  in  the  markings  of  the  petals  illumined  finger- 
posts to  lead  insects  to  the  hidden  hoards ;  and  he  further 
demonstrated,  that  in  some  bisexual  flowers  it  was  physically 
impossible  for  the  pollen  from  the  stamens  to  pass  to  the  tips 
of  the  carpels.  His  general  conclusion,  freely  stated,  was,  that 
"  since  a  large  number  of  flowers  have  the  sexes  separate,  and 
probably  at  least  as  many  hermaphrodites  have  the  stamens 
and  carpels  ripening  at  difl"erent  times,  nature  appears  to  have 
designed  that  no  flower  shall  be  fertilised  by  its  own  pollen." 
A  few  years  later  (1799),  Andrew  Knight  maintained  that  no 
hermaphrodite  flower  fertilises  itself  for  a  per[)etuity  of  gene- 
rations. 

Sprengel's  secret  of  nature  had,  however,  to  be  set  forth 
afresh  by  Darwin,  who,  in  his  "Fertilisation  of  Orchids" 
(1862),  and  "Effects  of  Cross-  and  Self-Fertilisation"  (1876), 
has  not  only  shown,  with  great  wealth  of  illustration,  the  mani- 


SEXUAL    REPRODUCTION. 


139 


fold  devices  for  ensuring  that  the  unconscious  insects  carry  the 
fertilising  pollen  from  one  flower  to  another,  but  has  also 
emphasised  the  beneficence  of  cross-fertilisation  for  the  health 
of  the  species.  "Nature  tells  us,"  he  says,  "in  the  most 
emphatic  manner  that  she  abhors  perpetual  self-fertiHsation." 
Hildebrand,  Hermann  Miiller,  Delpino,  and  others,  have,  with 
consummate  patience  of  observation,  further  traced  out  the 
secrets  of  nature  in  this  relation ;  and  the  student  may  be 
referred  to  Professor  D'Arcy  Thompson's  valuable  edition  of 
Miiller's  "  Fertilisation  of  Flowers,"  Sir  John  Lubbock's 
"Flowers  in  Relation  to  Insects,"  and  the  classic  works  of 
Darwin.  Reference  must,  however,  also  be  made  to  Meehan's 
protest  (see  pp.  75,  76),  that  self-fertilisation  is  neither  so  rare 
nor  so  "abhorrent"  as  is  now  generally  believed. 


Bees  visiting  White  Deadnettle  and  Broom. 

In  a  great  number  of  cases,  cross-fertilisation  by  means  of 
insects  does  occur  ;  in  many  it  must  occur.  In  another  by  no 
means  small  set  of  flowering  plants, — usually  with  inconspicuous 
blossoms, — the  fertilising  gold  dust  is  borne  by  the  wind,  and 
falls,  like  the  golden  shower  on  Danae,  upon  adjacent  flowers. 
In  many  hermaphrodite  flowers,  again,  self-fertilisation  does  cer- 
tainly take  place  ;  in  some  this  is  necessarily  so.  Interesting  in 
this  connection  is  the  indubitable  self-fertilisation  which  occurs 
in  the  small  degenerate  unopening  (cleistogamous)  flowers  of 
some  plants,  such  as  species  of  balsam,  deadnettle,  pansy,  &c. 
These  occur  along  with  ordinary  flowers,  and,  curiously  enough, 
are  sometimes  more  fertile  than  they. 

In  most  of  the  lower  plants,  the  male  elements  are  minute, 
and  actively  mobile.    They  find  their  way  through  the  water,  or 


140 


THE    EVOLUTION    OF    SEX. 


along  capillary  spaces  between  the  leaves,  to  the  passive  female 
cells.  In  some  cases  there  is  a  curvature  of  the  male  organ 
towards  an  adjacent  female  organ,  apparently  in  obedience  to 
chemical  or  physical  attraction.  Even  here  close  fertilisation 
seems  exceptional,  and  is  often  impossible. 

So  far,  however,  only  the  external  aspect  of  the  process. 
As  long  ago  as  1694,  Camerarius  showed  that  if  the  male 
flowers  of  hemp,  maize,  and  other  plants  were  removed,  the 
female  flowers  bore  no  seeds,  or  at  least  no  fertile  ones.  In 
1704,  E.  F.  Geoff"roy  castrated  certain  plants  by  removing  the 
stamens,  and  noted  that  they  remained  barren.  "  Mirandum 
sane,"  he  wrote,  "quam  similem  servet  natura  cunctis  in 
viventibus  generandis  harmoniam."  Reasonable  as  this  now 
appears  to  us,  the  fundamental  fact  was  not  only  slowly  recog- 
nised,   but   on   into    the    present   century   there   were   found 


A,  Enlarged  section  of  ripe  Anther  {/>),  liberating  pollen  (a).  J),  Diagrammatic 
section  of  a  Flower,  showing  female  parts  (c),— receiving  stigma,  conduct- 
ing style,  ovar^^  w  ith  seed  (cf)  ;  the  male  parts,  stamens  {/>)  with  pollen.  C, 
The  Pollen-tube  (a)  glowing  down  to  the  ovule  (</)and  female  cell  (e).  The 
pollen  grain  is  here  represented  as  distinctly  two-celled,  cf.  pp.  142  and  229. 

naturalists  who  strongly  opposed  it,  and  denied  the  sexuality  of 
plants  altogether.  In  1830,  however,  Amici  made  a  great  step. 
He  traced  the  pollen  grain  from  its  lighting  on  the  carpel  tip 
down  into  the  recesses  of  the  ovule.  Schleiden,  whose  name  is 
so  closely  associated  with  the  founding  of  the  "  cell  theory,"  soon 
confirmed  Amici's  observation,  but  in  doing  so  went  unfortunately 
much  too  far.  Not  only  did  the  pollen-grain  send  its  tube  into 
the  ovule,  but  there,  according  to  Schleiden,  it  gave  origin  to  the 
future  embryo.  This  opinion,  which,  as  Heyer  observes,  made 
the  male  element  really  female,  was  obviously  parallel  to  that  of 
the  zoologists  who  found  in  the  "sperm-animalcule"  the  mini- 


SEXUAL    REPRODUCTION. 


141 


ature  embryo.  The  view  of  Camerarius  and  Amici  of  course 
prevailed ;  and  we  now  know  not  only  the  fact  that  the  pollen- 
grain  is  a  male  element  which  unites  in  fertilisation  with  a 
female  cell,  but,  thanks  especially  to  Strasburger,  much  about  the 
intimate  nature  of  the  process.  In  the  last  century,  Millington 
emphasised  the  difference  between  male  and  female  flowers, 
and  we  can  trace  the  influence  of  this  discovery  in  Erasmus 
Darwin's  "  Loves  of  the  Plants." 

In  the  last  few  decennia,  it  has  been  shown,  for  many  of  the 
lower  plants,  that  fertilisation  essentially  involves  the  union  of 
the  nuclei  of  male  and  female  cells.  By  analogy  the  same  was 
believed  to  be  true  of  higher  plants,  but  direct  demonstration 
has  only  recently  been  forthcoming.     Strasburger  has  followed 


Illustrating  the  contrast  between  male  and  female 'flowers  in  the  pink 
campion  {^Lychnis  diii7-na). 

the  whole  history  of  the  pollen-grain,  from  the  anther  of  the 
stamen  to  the  embryo-sac  of  the  carpel ;  and  though  some  details 
still  remain  obscure,  his  researches  have  undoubtedly  succeeded 
in  elucidating  the  essential  facts  in  the  process.  He  shows 
how  the  pollen-grain  divides  into  a  vegetative  and  generative 
cell,  of  which  only  the  latter  is  directly  important  in  fertilisation. 
The  generative  cell,  which  consists  like  the  sperm  mostly 
of  nucleus  with  very  Htde  directly  associated  cell-substance, 
itself  divides  to  form  two  (or  even  more)  generative  nuclei. 
One  of  these  passes  from  the  pollen-tube  to  enter  into  close 
union  with  the  nucleus  of  the  female  cell,  with  which  it  fuses 
to  form  the  double  nucleus  ruling  the  forthcoming  develop- 
ment.    Exceptionally  the  other  generative  nucleus   may  also 


142  THE    EVOLUTION    OF    SEX. 

unite  with  the  nucleus  of  the  egg-cell,  but  this  is  almost  as  rare 
as  "  polyspermy"  among  animals.  According  to  Strasburger,  the 
cell-substance  of  the  pollen-grain  or  pollen-tube  which  surrounds 
the  nucleus  has  no  direct  influence  in  the  essential  act.  Fer- 
tilisation is  a  union  of  two  nuclei,  "  the  cell-substance  of  the 
pollen-tube  is  only  the  vehicle."  He  confirms  the  observations 
of  Pfeffer,  as  to  the  reality  of  an  osmotic  attraction  between  at 
least  the  surroundings  of  the  two  essential  elements,  in  accord- 
ance with  which  the  pollen-tube  bearing  the  generative  nucleus 
is  marvellously  guided  to  its  destination.  The  differentiation 
of  the  generative  nucleus,  in  contrast  to  the  more  vegetative,  and 
the  true  nuclear  union  which  forms  the  climax  of  fertilisation, 
are  two  very  important  facts,  showing  the  unity  of  the  process 
not  only  in  higher  and  lower  plants  but  in  all  organisms. 

§  4.  Fertilisatio7i  in  Ani??ials. — That  the  sperms  were  essen- 
tial to  fertilisation  was  a  conclusion  by  no  means  recognised 
when  those  elements  were  first  seen.  Gradually,  however,  the 
fact  was  demonstrated,  both  by  experiment  and  observation. 
Jacobi  (1764)  artificially  fertilised  the  ova  of  salmon  and  trout 
with  the  milt  of  these  forms,  and  somewhat  later  the  Abbe 
Spallanzani  extended  these  experiments  to  frogs  and  even  higher 
animals.  Even  he,  however,  believed  that  the  seminal  fluid 
was  the  essential  factor,  not  the  contained  spermatozoa.  Through 
the  experiments  of  Prevost  and  Dumas  (1824),  Leuckart  (1849), 
and  others,  attention  was  directed  to  the  real  import  of  the 
sperms,  which  Kolliker  referred  to  their  cellular  origin  in  the 
testes.  The  presence  of  the  sperm  within  the  ovum  was 
observed  in  the  rabbit  ovum  by  Martin  Barry  in  1843  ;  by 
Warneck,  in  1850,  for  the  water-snail,  a  fact  confirmed  about 
ten  years  afterwards  by  Bischoff  and  Meissner ;  in  the  frog 
ovum  by  Newport  (1854) ;  and  in  successive  years  it  was  gradu- 
ally recognised  in  a  great  variety  of  animals. 

The  external  devices  which  secure  that  the  sperms  shall 
reach  the  ova  are  very  varied.  Sometimes  it  seems  more  a 
matter  of  chance  than  of  device,  for  the  sperms  from  adjacent 
males  may  simply  be  washed  into  the  female,  as  in  sponges  and 
bivalves,  with  the  nutritive  water-currents.  In  other  cases, 
especially  well  seen  in  most  fishes,  the  female  deposits  her 
unfertilised  ova  in  the  water ;  the  male  follows  and  covers  them 
with  spermatozoa.  Many  may  have  watched  from  a  bridge  the 
female  salmon  ploughing  along  the  gravelly  river  bed  depositing 
her  ova,  careful  to  secure  a  suitable  ground,  yet  not  disturbing  the 


SEXUAL    REPRODUCTION. 


143 


already  laid  eggs  of  her  neighbours.  Meanwhile  she  is  attended 
by  her  (frequently  much  smaller)  mate,  who  deposits  milt  upon  the 
ova.  In  the  frog,  again,  the  eggs  are  fertilised  externally  by  the 
male  just  as  they  leave  the  body  of  his  embraced  mate.  Or  it 
may  be  that  the  sperms  are  lodged  in  special  packets,  which  are 
taken  up  by  the  female  in  most  of  the  newts,  surrounded  with 
one  of  the  male  arms  in  many  cuttle-fishes,  or  passed  from 
one  of  the  spider's  palps  to  the  female  aperture.  In  the  majority 
of  animals,  <?.,^.,  insects  and  higher  vertebrates,  copulation  occurs, 
and  the  sperms  pass  from  the  male  directly  to  the  female. 
Even  then  the  history  is  very  varied.  They  may  pass  into 
special  receptacles,  as  in  insects,  to  be  used  as  occasion  demands; 
or,  in  higher  animals,  they  may  with  persistent  locomotor  energy 
work  their  way  up  the  female  ducts.  There  they  may  soon 
meet  and  fertilise  ova  which  have  been  liberated  from  the  ovary  ; 
or  may  persist,  as  we  noticed,  for  a  prolonged  period ;  or  may 
eventually  perish. 


c    ''  //        '/  "    ff 

Different  Forms  of  Conjugation  in  Plants. 
a,  zoospores  ;  h,  mould  ;  c,  d,  conjugate  alga; ;  e,f,  desmid. 

When  the  sperms  have  come,  in  any  of  these  varied  ways, 
into  close  proximity  to  the  ovum,  there  is  every  reason  to 
believe  that  a  strong  osmotic  attraction  is  set  up  betw^een  the 
two  kinds  of  elements.  We  have  often  suspected  that  the 
approach  of  the  conjugating  cells  of  two  Spirogyra  filaments 
(fig.  r,  d)  might  be  directed  along  the  line  of  an  osmotic  current ; 
and  although  we  must  confess  that  perhaps  somewhat  rough 
evaporations,  performed  a  few  summers  ago,  gave  no  positive 
confirmation   to   the   idea    that   glucose  or  the  like  might  be 


144  THE    EVOLUTION    OF    SEX. 

present  in  appreciable  quantity  in  the  water,  a  recent  observer, 
we  are  glad  to  see,  claims  to  have  been  more  fortunate.  The 
spermatozoa,  which  seem  so  well  to  deserve  Rolph's  epithet  of 
"starved,"  appear  to  be  powerfully  drawn  to  the  well-nourished 
ovum,  and  the  latter  frequently  rises  to  meet  the  sperm  in  a 
small  "attractive  cone."  Often,  however,  there  is  an  obstacle 
in  the  way  of  entrance  in  the  form  of  the  egg-shell,  which  may 
be  penetrable  only  at  one  spot,  well  called  the  micropyle. 
Dewitz  has  made  the  interesting  observation,  that  round  the 
egg-shells  of  cockroach  ova,  the  sperms  move  in  regular  circles 
of  ever  varying  orbit ;  and  points  out  that  thus,  sooner  or  later,  a 
sperm  must  hit  upon  the  entrance.  He  showed  that  this  was  a 
characteristic  motion  of  these  elements  on  smooth  spheres,  for 
round  empty  egg-shells  or  on  similar  vesicles  they  moved  in  an 
equally  orderly  and  systematic  fashion.  It  was  till  recently 
believed  that  more  than  one  sperm  might  at  least  enter  the 
ovum,  but  researches  such  as  those  of  Hertwig  and  F'ol  have 
shown  that  when  one  sperm  has  found  admittance,  the  way  is 
usually  barred  against  others.  The  micropyle  may  be  blocked, 
or  the  surrounding  membrane  may  be  altered,  or  in  other  ways 
the  ovum  may  exhibit  what  Whitman  calls  "self-regulating 
receptivity,"  so  as  to  be  no  longer  penetrable.  We  are  safe  in 
concluding, — that  the  ovum  is  usually  receptive  only  to  one 
sperm  ;  that  in  most  cases  the  entrance  of  more  than  one  sperm 
is  impossible  ;  and  that  where  "  polyspermy  "  does  occur,  patho- 
logical development  is  at  least  often  the  result.  In  the  lamprey's 
egg  quite  a  number  of  sperms  find  their  way  into  a  watch-glass- 
shaped  space  at  the  upper  pole  of  the  ovum,  but  only  one  gets 
further,  the  rest  remain  imprisoned  without  further  history  of 
any  importance. 

What  takes  place  before  fertilisation  is,  as  we  have  just  seen,  very 
varied  indeed  among  animals  ;  what  takes  place  after  fertilisation  is  of 
course  cell-division,  but  that,  though  referable  to  certain  great  types,  must 
necessarily  vary  with  each  species  ;  what  takes  place  in  the  act  of  fertilisa- 
tion, however,  is  always  essentially  the  same.  The  head  of  the  sperma- 
tozoon becomes  the  male  nucleus  (or  pro-nucleus)  of  the  fertilised  ovum, 
entering  into  close  association  with  the  female  nucleus.  The  latter,  as  we 
have  already  noted,  has  had  its  own  history  ;  it  is  no  longer  the  original 
germinal  vesicle,  nor  usually  like  it  in  appearance,  it  is  the  germinal  vesicle 
minus  the  cjuantity  of  nuclear  substance  given  off  in  forming  two  polar 
globules.  This  female  nucleus  (or  pro-nucleus,  as  it  is  generally  called) 
comes  into  close  association  with  the  sperm  or  male-nucleus  ;  nor  does  it 
remain  quite  passive  in  the  process,  though  the  greater  activity  in  bringing 
about  the  close  association  is  certainly  still  exhibited  by  the  male.     Whit- 


SEXUAL    REPRODUCTION.  I45 

man  has  recently  emphasised  the  reahty  of  an  attractive  influence  between 
the  pro-nuclei.  P\ision  of  the  pro-nuclei  was  observed  so  long  ago  as  1850 
by  Warneck  in  the  pond-snail  [Lyinuccus).  This  result,  however,  appears 
to  have  been  overlooked,  till  the  same  fact  was  reobserved  in  threadworm 
ova  by  Butschli  in  1874.  Since  that  date  the  fact  has  been  continuously 
studied.  Some  observers  still  doubt  whether  what  can  be  accurately  called 
fusion  of  nuclei  ever  occurs  ;  and  if  fusion  means  inextricable  confounding 
and  mixing  up  of  the  male  and  female  nuclear  elements,  it  is  almost  certain 
that  such  does  not  in  any  case  happen.  There  is  no  doubt,  however,  that 
the  two  nuclei  become  very  closely  associated,  and  according  to  most 
observers  a  double  unity  is  formed,  in  wdiich  the  component  nuclear 
elements  from  the  tw^o  origins  so  diverse  are  united  in  perfectly  orderly 
fashion.  So  exact,  in  fact,  is  this  duality,  that  v/hen  the  first  division  of  the 
egg  takes  place,  each  of  the  two  daughter-cells  has  in  its  nucleus  half  of  the 
male  and  half  of  the  female  elements,  and  so  on  perhaps  in  after-stages. 

The  object  upon  which  the  intimate  phenomena  of  fertilisation  have  been 
most  studied  is  the  ovum  of  the  threadworm  {Ascaris  uiegalocepJiaJd)  which 
infests  the  horse.  Since  1883  about  a  dozen  important  memoirs  have  dealt 
with  this  subject,  and  with  the  same  material.  The  results  of  competent 
observers  have  varied  enormously  in  detail,  but  on  the  essential  points  there 
has  been  (with  some  few  exceptions)  an  increasing  congruence  of  opinion. 
The  most  important  work  on  the  subject  has  been  that  of  Prof,  van  Beneden, 
whom  most  of  those  who  have  followed  him  unite  in  regarding  as  a  master. 
The  discrepancies  and  contradictions  have  been  accompanied  at  times  by 
not  a  little  warmth  of  asseveration,  but  with  ever-increasing  perfection  of 
method  many  of  these  are  disappearing.  To  one  alone  shall  we  here  allude. 
According  to  Van  Beneden,  the  normal  ovum  of  this  threadworm  contained 
in  its  nucleus  one  chromatin  element,  and  was  fertilised  by  a  sperm  also 
with  one  chromatin  element.  Carnoy,  however,  described  the  normal 
ovum  as  containing  two  chromatin  elements,  and  as  fertilised  by  a  sperm 
also  with  two.  In  view  of  the  perfection  with  which  both  these  investi- 
gators had  unravelled  the  structure  and  behaviour  of  the  nuclei,  the  dis- 
crepancy seemed  serious  enough.  Now,  however,  Boveri  has  shown  that 
both  are  right  ;  Van  Beneden's  type  occurs  ;  Carnoy's  type  also  occurs. 
Nay  more,  an  ovum  with  one  chromatin  element  seems  to  be  always 
fertilised  by  a  sperm  with  only  one,  while  an  ovum  with  two  chromatin 
elements  is  fertilised  by  a  sperm  likewise  with  two. 

A  few  of  the  details  may  be  summarised  from  the  recent  masterly 
researches  of  Boveri.  The  extrusion  of  the  two  polar  cells  from  the  ovum 
is  in  reality  a  double  process  of  cell-division.  The  quantity  of  the  nuclear 
substance  in  the  germinal  vesicle  is  thereby  reduced  by  three-fourths,  but 
the  number  of  nuclear  elements  remains  the  same.  Only  one  sperm  pene- 
trates the  ovum,  unless  the  latter  be  unhealthy  ;  and  with  the  entrance  of 
the  sperm  the  ovum  undergoes  a  simultaneous  change,  which  excludes  other 
male  elements.  Only  the  head  or  nuclear  portion  of  the  sperm  is  of  real 
importance  in  the  essential  act  of  fertilisation  ;  the  nutritive  tail  or  cap 
simply  dissolves  away.  After  the  sperm-nucleus  has  penetrated^  to  tlie 
centre  of  the  ovum,  and  after  the  extrusion  of  the  polar  bodies  is  quite 
completed,  we  have  to  deal  with  two  nuclei,  not  only  closely  approximate 
in  structure,  but  alike  in  further  history. 

In  Carnoy's  type,  both  male  and  female  nuclei  contain  two  chromatin 
elements,  in  the  form  of  bent  rods;  and  before  union  takes  place,  these 

K 


146 


THE    EVOLUTION    OF    SEX. 


undergo  a  marked  modification,  the  same  in  both  cases.  Round  the  chro- 
matin rods  vacuoles  are  formed,  limiting  them  from  the  surrounding 
protoplasm  ;  into  these  the  rods  send  out  anastomosing  jirocesses,  after  the 
fashion  of  little  rhizopods  ;  gradually  the  rods  thus  resolve  themselves  into 
a  network,  in  the  meshes  of  which  minute  "  nucleoli  "  are  also  demonstrable. 

T  I 


Diagram  of  the  Process  of  Fertilisation,  following  Eoveri's  figures. — n,  female  pro- 
nucleus ;  /',  polar  bodies  ;  r,  male  nucleus  ;  d,  sperm-cap  ;  ac,  chromatin  elements 
of  uniting  united  female  and  male  nuclei  {a  and  c)  ;  c,  protoplasmic  centres  ;  yj 
archoplasniic  threads. 

The  two  nuclei  thus  modified  then  unite,  but  that  again  so  precisely,  as 
Van  Benedcn  esi)ecially  has  shown,  that  each  forms  half  of  that  spindle 
figure  which  almost  all  nuclei  take  when  about  to  divide.  This  double 
spindle  figure  is  the  "segmentation  nucleus,"'  which  will  presently  divide 
into  the  two  first  daughter-nuclei  of  the  ovum  (see  figs.  VL-X.). 

It  is  not  possible  here  to  discuss  certain  intricate  changes  which  take 
place  meanwhile,  not  in  the  nuclei,  but  in  the  cell-substance  of  the  ovum. 
Both  Van  Bencden  and  Boveri  have  recently  agreed  on  the  existence  of  two 
"central  corpuscles"  (centrosomata)  in  the  proto]:)lasm,  which  serve  as 
"  points  of  insertion  "  for  protoplasmic  threads,  which  exert  a  "muscular 
action  "  upon  the  nuclear  elements  in  the  forthcoming  division.      Boveri 


SEXUAL    REPRODUCTION. 


M7 


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148  THE    EVOLUTION    OF    SEX. 

has  traced  with  great  care  the  history  of  a  special  kind  of  protoplasm  (what 
he  calls  thearchoplasm),  which  has  its  centre  in  either  "central  corpuscle "(^), 
and  sends  out  contractile  fibrils  {/)■,  which  moor  themselves  to  the  nuclear 
elements.  The  movements  of  the  latter  during  the  forthcoming  first 
division  of  the  ovum  are  directly  referable  to  the  antagonistic  action  of 
these  fibrils,  and  thus  we  have  hints  of  an  intracellular  muscularity,  the 
thought  of  which  makes  one  dumb. 

In  the  spindle  the  nuclear  elements,  still  distinguishable  in  their  orderly 
behaviour  as  male  and  female,  eventually  form  what  is  known  as  the  "equa- 
torial plate"  (VI.),  lying  across  the  centre  of  the  spindle.  This  is  a  well-marked 
stage,  and  one  characterised  by  apparent  equilibrium.  "It  is  the  resting- 
stage/ar  excellence  in  the  life  of  the  cell.  Movement  is  at  an  end,  a  state 
of  stability  has  set  in,  and  this  would  continue  ad  iiifniilii/n,  did  not  a  factor, 
which  hitherto  has  played  no  part,  assert  itself  and  bring  about  fresh  move- 
ment. This  new  movement  is  the  longitudinal  division  of  the  chromatin 
elements,  an  independent  expression  of  life — indeed,  a  reproductive  act — 
on  the  part  of  the  nuclear  elements." 

»The  above  short  sketch  wih  show  how  intricate,  and  yet  at 
the  same  time  how  orderly,  are  the  intimate  processes  of  fer- 
tihsation.  Variations  do  indeed  occur,  both  in  pathological 
and  in  apparently  normal  cases  ;  but  a  general  constancy  is  now 
both  clear  and  certain,  not  only  for  many  different  animals,  but 
also  to  a  certain  extent,  as  Strasburger  has  shown,  for  plants. 

One  marvellous  fact,  showing  the  closeness  of  union  in  fer- 
tilisation, may  be  briefly  re-emphasised.  In  the  double  nucleus 
formed  from  the  union  of  male  and  female  nuclei,  Van  Beneden, 
Carnoy,  and  others,  have  shown  that  both  constituents  have  an 
equal  share.  "  The  one  half  is  purely  male,  the  other  purely 
female,  and  this  is  true  not  only  for  Ascaris  (by  Van  Beneden) 
and  other  thread-worms  (by  Carnoy),  but  for  representatives  of 
other  worm-types,  coelenterates,  echinoderms, molluscs,  and  tuni- 
cates."  In  division  to  form  daughter-cells  (IX.,  X.),  half  of  each 
set  of  constituents  goes  to  either  cell,  and  the  dualism  is  kept  up. 
Furthermore,  though  hardly  yet  quite  certain,  it  is  most  probable, 
that  "of  the  four  chromatin  loops  observed  in  the  division  figure 
of  a  daughter-cell,  two  are  derived  from  the  male  parent,  and  two 
from  the  female. ""  The  importance  of  this  fact,  in  relation  to  the 
influence  of  both  parents  upon  the  offspring,  is  very  obvious. 

§  5.  Fertilisation  in  Protozoa. — In  the  nascent  sexual  union  observed  in 
many  Protozoa, — not,  however,  as  yet  in  foraminifers  or  radiolarians, — con- 
siderable diversity  obtains.  The  individuals  which  unite  may  be  to  all 
ajipearance  similar  (to  which  cases  the  term  conjugation  is  generally  applied), 
or  they  may  l)e  materially  dimorphic,  as  in  Vorticella.  The  union  may  be 
permanent,  when  the  two  units  fuse  into  one;  or  it  may  only  be  temporary, 
during  which  an  interchange  of  elements  takes  place.  In  both  cases  the 
nuclear   elements    play    an   important  part,   disrupting  and  reconstructing 


SEXUAL    REPRODUCTION.  I49 

(luring  the  process,  while  a  genuine  fusion  of  the  two  nuclei  has  also  been 
observed  in  permanent  conjugation. 

In  regard  to  the  interchange  of  elements,  there  is  considerable  diver- 
gence of  observation.  Joseph  has  noted  what  appears  to  be  an  interchange 
of  protoplasm  ;  Schneider  has  observed  the  exchange  of  nuclear  elements  ; 
while  Grul:)cr  and  Maupas,  and  Joseph  as  well,  have,  in  their  studies  on  the 
union  of  ciliated  infusorians,  laid  emphasis  on  an  accessory  nuclear  body, 
generally  known  as  the  "paranucleus."  This  body  lies  by  the  side  of  the 
larger  nucleus,  and  while  the  latter  simply  disrupts  and  dissolves  away,  or 
is  extruded  without  playing  any  important  part,  the  smaller  paranucleus 
divides  in  a  regular  way,  and  with  the  results  there  is  interchange  between 
the  two  individuals. 

According  to  Maupas,  who  has  investigated  the  subject  in  most  detail, 
the  para- or  micro-nucleus  is  a  "hermaphrodite"  sexual  element,  of  sole 
importance  in  conjugation.  The  stages  in  the  process  of  fertilisation  are  as 
follows  :  — 

(i.)  The  para-nucleus  increases  in  size. 

(2,  3.)  It  then  divides  twice,  and  eliminates  certain  corpuscles. 

(4.)  This  effected,  it  divides  again,  differentiating  a  male  and  female 

pro-nucleus. 
(5.)  In  the  next  stage,  the  male  elements  of  the  two  individuals  are 
exchanged,  and  the  new  male  nucleus  fuses   with   the  original 
female  ])ortion. 
(6,  7.)  In  two  following  stages,  the  nuclear  dualism  characteristic  of  the 
ciliated   infusorians  is   re-eslablished.      The  old    large    nucleus 
(macro-nucleus)  has  broken  up  and  been  eliminated  meanwhile. 
(8.)  Finally,  the  individuals,  separating  from  one  another,  reassume  all 
their  original  organisation  before  beginning  again  to  divide  in  the 
usual  fashion. 
The  union  of  the  male  and  female  nuclear  elements  in  ciliate  infusorians 
was  admirably  figured  by  Balbiani  so  long  ago  as  1858  ;  and  though  he  does 
not  seem  rightly  to  have  interpreted  what  he  oljserved  in  this  particular  case, 
he  was  right  in  his  contention   that   sexual   union   and   fertilisation  really 
occurred  in  the  Protozoa.      Balbiani's  view  has  been^for  long  scouted,  and 
yet,  with  renewed  ol)servation,  naturalists  have  now  come  back  to  his  con- 
clusion.    Maupas  willingly  allow^s  that  Balbiani  figured  beautifully  what  he 
himself  has  since  reobserved  and  interpreted. 

The  phenomena  described  by  Maupas,  as  summarised  above,  have  been 
observed  in  towards  a  dozen  ciliated  infusorians,  so  that  there  is  every  reason 
to  believe  in  their  general  occurrence.  In  three  species  of  the  slipper  ani- 
malcule {Paraiii(rLiti//i),  and  in  species  oi  StylonicJiia^  Leitcophrys,  Eziplotes, 
0)iycJiodro))nts,  Spirostonnini,  &c.,  the  facts  are  as  above  stated. 

It  is  of  interest  to  cite  the  facts  in  regard  to  the  common  bell-animalcule 
(  ^; //rt'//a), because  here  the  conjugating  individuals  arelikeovum  and  sperm 
in  more  ways  than  one.  In  some  species — e.g.^  V.  inoiiilata — the  adult 
divides  equally,  to  form  two  small  individuals,  which  conjugate  with  those 
of  normal  size.  In  V.  microstoma  there  is  again  division  into  two,  but  the 
products  are  of  unequal  size  ;  one  is  more  male  than  the  other.  In  the 
nearly  allied  Carc/iesiiim polypvnim,  the  divisions  are  equal,  Init  they  are 
repeated  twice  or  thrice.  The  result  in  all  cases  is  the  production  of 
minute  individuals,  which  eventually  attach  themselves  to  adults  of  the  nor- 
mal size,  first  to  the  stalk,  and  then  to  the  liody  (fig.  p.  129).     The  accessory 


150  THE    EVOLUTION    OF    SEX. 

nuclear  bodies  divide  as  usual  ;  the  large  individual  ceases  to  feed,  and 
hermetically  closes  its  mouth,  like  an  ovum  when  fertilised.  The  small 
individual  is  gradually  absorbed  by  the  larger,  as  sperm  by  ovum  ;  and  in 
an  intricate  but  orderly  fashion  a  mixed  nucleus  results  from  the  fusion  of 
the  para-nuclear  elements  of  the  rwo.  The  adult  then  begins  to  feed,  to 
divide,  and  so  on,  as  usual.  Here  then  there  is  {a)  incipient  dimorphism,  {/>) 
absorption  of  smaller  by  larger,  and  {c)  intimate  nuclear  union, — facts  which 
we  have  already  emphasised  in  the  fertilisation  of  multicellular  animals. 

§  6.  Origin  of  Fertilisation. — To  understand  the  origin  of 
the  union  of  sex-cells,  attention  must  still  be  concentrated  on 
the  Protozoa.  That  fertilisation  really  occurs  at  that  low  level 
in  a  highly  complex  fashion,  we  have  just  seen.  It  is  necessary, 
however,  to  note  the  steps  which  lead  up  to  what  Maupas  and 
others  have  so  patiently  elucidated. 

{a)  In  the  primitive  life-cycle  exhibited  by  Protoniyxa  (see 
fig.  at  p.  120),  the  units  which  burst  forth  from  the  cyst  sink 
down  into  tiny  amoeboe,  and  unite  together  in  numbers  to  form 
a  composite  spreading  mass  of  protoplasm,  technically  known 
as  a  plasniodiin?i.  This  is  undoubtedly  a  very  primitive  union 
of  cells,  yet  it  occurs  at  very  diverse  levels  in  the  organic  series. 
It  is  more  or  less  familiar  in  the  "flowers  of  tan,"  one  of  the 
lowly  Myxomycetes,  where  a  nucleated  mass  of  protoplasm, 
of  composite  origin,  spreads  over  the  bark  in  the  tan-yard. 
The  plasmodial  union  also  occurs  as  a  definite  stage  in  the  life- 
history  of  the  primitive  neighbours  of  P?'otoniyi\a,  the  Monera 
of  Hceckel.  Pour  the  liquid  contents  or  body-cavity  fluid  of  a 
freshly-dredged  and  still  actively  living  sea-urchin  into  a  bowl ;  the 
cells  which  float  in  it,  like  blood-corpuscles  in  the  blood,  draw 
together  in  clotted  masses.  Watch  the  process  under  a  micro- 
scope, and  the  formation  of  a  plasmodium  is  seen.  The  dying 
cells  fuse  into  composite  masses,  just  like  the  units  oi  Protoniyxa  ; 
and  it  is  interesting  to  observe  that,  though  they  are  dying,  the 
union  provokes  a  brief  but  intense  renewal  of  amoeboid  activity. 
To  forestall  our  point,  they  as  it  were  fertilise  each  other  in 
articulo  mortis.  In  spite  of  the  objection  of  Michel  and  others, 
that  such  union,  being  pathological,  is  not  comparable  to  the 
multiple  conjugation  normal  to  the  myxomycete,  we  maintain 
the  distinct  analogy  between  the  plasmodium  formation  in 
myxomycetes  and  that  exhibited  by  the  cells  in  the  body-cavity 
fluid  of  many  animals,  and  regard  this  as  so  much  additional 
evidence  of  the  i)rofound  unity  of  the  normal  and  the  patho- 
logical processes.  Now  it  is  from  this  primitive  union  of  cells, 
as  illustrated  in  the  lowest  organisms,  that  we  start  in  explain- 


SEXUAL    REPRODUCTION. 


151 


ing  the  origin  of  fertilisation.  Just  as  the  very  beginning  of 
reproduction  may  be  detected  in  the  ahnost  mechanical  break- 
age of  a  form  like  Sc/iizogenes,  so  the  very  beginning  of  fertili- 
sation is  found  in  the  almost  mechanical  flowing  together  of 
exhausted  cells. 


^r 


Diagrammatic  representation  of  the  stages 
in  the  origin  of  fertilisation, — (I.)  Plas- 
modium ;  (II-)  multiple  conjugation; 
(III.)  ordinary  conjugation;  (IV.)  con- 
jugation of  dimorphic  cells  ;  (V.)  fertilis- 
ation of  ovum  by  spermatozoon. 

(b)  Between  this  and  the  process  usually  described  as  con- 
jugation, there  are  some  interesting  links.  Sometimes  as  many 
as  three  or  four  spores  of  lowly  Alg?e  club  together,  as  if  to 
gather  sufficient  momentum  to  make  a  combined  start  in  life. 
The  young  forms  of  the  sun-animalcule  iyActijwsphcBriwii) 
usually  unite  in  twos,  but  Gabriel  has  observed  in  some  cases 
a  multiple  union.  So  in  gregarines  (common  parasites  in 
invertebrates),  while  the  usual  union  is  certainly  dual,  Gruber 
has  again  observed  what  may  be  termed  multiple  conjugation. 
Union  of  three  has  also  been  observed  as  an  exception  in 
several  infusorians.  The  union  of  more  than  two  may  thus  be 
interpreted  as  intermediate  between  the  formation  of  plasmodia 
and  the  normal  dual  conjugation. 

{c)  Conjugation  of  two  5-//////^';"  unicellular  organisms  occurs, 


152 


THE    EVOLUTION    OF    SEX. 


as  we  have  seen,  very  generally  in  the  Protozoa,  and  is  also  a 
common  fact  in  the  life-history  of  simple  Algae.  It  is  open  to 
every  one  possessed  of  a  microscope  to  observe  what  conjuga- 
tion means  in  such  a  common  fresh-water  alga  as  Spirogyra. 
Opposite  cells  of  adjacent  filaments  are  attracted  to  one  another 
by  what  a  recent  observer  calls  a  "  purely  physical  process,"  and 
the  contents  of  the  one  cell  pass  bodily  over  into  the  other. 
In  the  great  majority  of  cases  where  conjugation  occurs,  the 
uniting  cells  are  to  all  appearance  similar,  but  it  must  be 
remembered  that  it  does  not  follow  from  this  that  they  are 
physiologically  alike  (see  fig.  p.  143). 


'^■. 


Diagrammatic  representation  of  the  contrast 
between  conjugation  (horizontal  line)  and 
fertilisation  (vertical  line). 

{d)  Both  among  plants  and  animals,  all  naturalists  are 
agreed  that  it  is  impossible  to  draw  any  line  between  the  con- 
jugation of  similar  and  the  union  of  more  or  less  dimorphic 
elements.  "This  differentiation  presents,"  Sachs  says,  "especi- 
ally in  Algse,  a  most  complete  series  of  gradations  between  the 
conjugation  of  similar  cells  and  the  fertilisation  of  oospheres  by 
antherozoids,  any  boundary  line  between  these  two  processes 
being  unnatural  and  artificial."  The  gradual  appearance  of 
dimorphism  has  been  already  noted  in  discussing  the  origin 
of  sex,  and  need  not  be  re-emphasised. 

{e)  Lastly,  in  fertilisation  among  higher  plants  and  animals, 
the  two  elements  which  unite  are  highly  differentiated,  alike  in 
contrast  to  one  another  and  in  opposition  to  the  general  cells 
of  the  body.  A  consideration  of  the  phenomena  in  loose  pro- 
tist  colonics  like  Volvox  or  Ampullina,  which  suggest  the  bridge 


SEXUAL    REPRODUCTION.  153 

between  unicellular   and  multicellular   organisms,    shows    how 
gradually  this  latter  contrast  also  may  have  been  brought  about. 
To  sum  up,  the  steps  in  the  development  of  the  process  of 
fertilisation  may  be  arranged  in  the  following  series : — 

(a)  The  formation  of  plasmodia. 

(If)  Multiple  conjugation. 

(<r)  Conjugation  of  two  similar  cells. 

(^)  Union  of  incipiently  dimorphic  cells. 

(e)  Fertilisation  by  differentiated  sex-elements. 
One  difficulty  must  in  fairness  be  allowed  in  connection 
with  the  hypothesis  of  deriving  conjugation  from  plasmodial 
union.  Some  years  ago,  Sachs  was  inclined  to  regard  the 
Plasmodium  formation  of  Myxomycetes  as  a  process  of  multiple 
conjugation,  but  has  since  withdrawn  this  view  mainly  on  the 
ground  that  the  nuclei  have  not  been  shown  to  coalesce.  Now 
there  seems  no  result  of  studies  on  fertilisation  more  certain 
than  that  the  union  of  nuclei  is  an  essential  fact,  but  in  plasmo- 
dium-formation,  such  intimate  association  of  nuclei  cannot  be 
asserted.  The  difficulty  of  making  this  a  starting-point  is  thus 
at  first  sight  considerable. 

Yet  it  must  be  observed,  (i)  that  our  knowledge  of  the 
nuclei  in  those  lowly  forms  is  still  very  inadequate;  (2)  that, 
according  to  Gruber,  the  behaviour  of  the  nucleus  is  sometimes 
masked  by  the  fact,  that  instead  of  existing  as  a  discrete  body 
in  the  cell,  it  lies  diffusely  in  the  protoplasm  ;  but  especially 
(3)  that  it  is  quite  consistent  with  the  general  evolutionary  con- 
ception to  suppose  that  the  primitive  union  was  of  very  much 
less  definite  character  than  that  subsequently  evolved.  A 
reinvestigation  of  the  whole  question  of  plasmodium  formation, 
from  this  point  of  view,  is  however  very  desirable,  especially 
since  the  recent  progress  of  microscopic  technique  has  rendered 
the  study  of  the  nucleus  in  the  lowest  forms  much  more  practi- 
cable than  it  was  a  few  years  ago. 

§  7.  Hybridisation  in  Ani)iials. — Many  of  the  compound  names  of 
animals,  such  as  leopard,  point  back  to  a  once  prevalent  belief  that  animals 
of  very  different  kinds  might  unite  sexually  and  have  fertile  offspring.  Only 
lo  a  very  limited  extent  is  such  a  notion  justified.  Every  one  is  aware  that 
by  direct  human  control  animals  like  horse  and  ass,  dog  and  wolf,  lion  and 
tiger,  hare  and  rabbit,  canary  and  finch,  pheasant  and  hen,  goose  and  swan, 
N^  have  been  successfully  crossed.  In  nature,  however,  we  know  very  little 
X  of  the  occurrence  of  any  such  hybridisation.  It  seems  to  occur  in  some 
\ishes  ;  different  species  of  toad  are  often  seen  in  sexual  union,  but  the 
result  is  unknown  ;  in  higher  animals  it  seems  confined  to  varieties  of  a 
species.     The  demarcation  of  a  species  is  the  vague  line  which  marks  the 


154  THE    EVOLUTION    OF    SEX. 

physiological  range  of  natural  and  successful  intercrossing.  Domesti- 
cated breeds  are  usually  fertilisable  mutually,  and  their  progeny  is  fertile  ; 
we  regard  them  as  mere  varieties.  Nearly  related  species,  however,  only 
rarely  admit  of  being  crossed,  even  when  under  man's  control ;  and  species 
hybrids  tend  to  be  themselves  sterile.  In  structural  characters  two 
varieties  of  dog  may  seem  more  widely  separate  than  two  nearly  allied 
species,  yet  the  varieties  of  dog  may  be  intercrossed,  while  this  very 
rarely  occurs  with  the  two  species.  The  difference  in  the  reproductive 
elements  must  often  Ije  greater  than  the  structural  differences  of  the 
atlults  would  suggest.  Hertwig  has  experimented  of  late  with  the  artificial 
hybridisation  of  echinoderms,  and  Born  with  that  of  amphibians.  Both 
emphasise  the  difficulties  of  the  process,  and  the  varying  degrees  of  success 
that  may  result.  In  three  cases  Born  brought  about  reciprocal  hybridisa- 
tion, but  this  is  by  no  means  always  the  case.  Sometimes  real  fertilisation 
took  place,  but  nothing  followed  ;  in  other  cases  the  ova  segmented  ;  in  a 
few  the  larval  stages  were  reached  ;  and  in  two  cases  metamorphosis  was 
survived.  The  hybridisation  is  the  more  readily  effected  the  nearer  the 
elements  are  to  perfect  maturity.  Sometimes  the  success  seemed  greatly 
to  depend  on  the  concentration  of  the  sperm  fluid, — the  more  dilute  this 
was,  the  fewer  sperms  were  there  to  overcome  the  difficulties  of  effecting 
entrance  to  the  ovum. 

There  is  no  doubt  that  at  least  many  species-hybrids  tend  to  sterility,  but 
this  is  exhibited  in  varying  degrees.  The  male  mules  are  always  sterile,  but 
the  females  may  be  successfully  impregnated  by  horse  or  ass.  In  many  cases 
the  hybrids  are  not  fertile  with  one  another,  l)ut  remain  so  with  the  parent 
form.  In  a  few  cases  the  reproductive  functions  seem  for  a  time  at  least 
to  be  exaggerated  rather  than  diminished  as  the  result  of  crossing.  It 
seems  also  certain  that  while  variety-hybrids  are  usually  fertile,  their  con- 
stitution is  more  or  less  unstable.  They  are  often  very  variable,  and  apt  to 
die  out,  as  has  been  repeatedly  observed  in  the  human  species.  The  ill- 
natured  saying,  "  God  made  the  white  man,  God  made  the  black  man,  the 
devil  made  the  mulatto,"  refers  to  the  frequently  inconvenient  variability  of 
those  variety-hybrids. 

Brooks  has  laid  considerable  emphasis  on  the  variability  of  hybrids  in 
connection  with  his  theory  of  heredity.  "  Hybrids  and  mongrels,"  he 
says,  "  are  highly  variable,  as  we  should  expect  from  the  fact  that  many  of 
the  cells  of  their  bodies  must  be  placed  under  unnatural  conditions,  and  must 
therefore  have  a  tendency  to  throw  oftgemmules."  "  Hybrids,  from  forms 
which  have  been  long  cultivated  or  domesticated,  are  more  variable  than 
those  from  wild  species  or  varieties,  and  the  children  of  hybrids  are  more 
variable  than  the  hybrids  themselves."  "  But  domesticated  animals  and 
plants  live  under  unnatural  conditions,  and  they  are  therefore  more  prolific 
of  gemmules  than  wild  species  ;  and  as  the  body  of  a  male  hybrid  is  a  new 
thing,  the  cells  will  be  much  more  likely  than  those  of  the  pure  parent  to 
throw  off  gemmules.  The  fact  that  variation  is  due  to  the  male  influence, 
and  that  the  action  upon  the  male  parent  of  unnatural  or  changed 
conditions  results  in  the  variability  of  the  child,  is  well  shown  by  crossing 
the  hybrid  with  the  pure  species  ;  for  when  the  male  hybrid  is  crossed  with 
a  pure  female,  the  children  are  much  more  variable  than  those  born  from  a 
hybrid  mother  by  a  pure  father." 

When  we  regard  the  male  as  katabolic,  his  variability  becomes  intel- 
ligible;  while  in  hybridisation,  which  means  the  sexual  union  of  organisms 


SEXUAL    REPRODUCTION.  155 

with  a  more  than  usually  divergent  life-experience,  the  reproductive 
elements  which  are  intermingled  have  probably  a  corresponding  divergence 
in  chemical  constitution. 

The  early  researches  of  Kolreuter  (1761)  gave  a  firm  basis  to  the  study 
of  hybridisation  among  plants.  The  comparative  easiness  of  experiment 
has  advanced  the  botanical  side  of  the  subject  to  far  greater  certainty  than  the 
zoological  conclusions  can  pretend  to.  Among  plants,  as  we  should  expect 
from  their  greater  vegetativeness,  the  fertility  of  hybrids  seems  frequently 
established.  Knight,  Gartner,  Herbert,  Wichura,  and  others,  have 
brought  together  a  great  number  of  reliable  observations,  and  the 
whole  subject  has  been  admirably  discussed  by  Niigeli.  For  a  copious 
resume  of  the  general  results,  for  the  most  part  after  N'ageli,  the  student 
may  be  referred  to  chap.  vi.  of  Sachs'  Text-book  of  Botany,  while  Wallace's 
"  Darwinism"  should  be  consulted  for  its  rediscussion  of  hybridisation  in 
animals. 


156  THE    EVOLUTION    OF    Sr:X. 


SUMMARY. 

1.  Reproduction  is  hut  more  or  less  discontinuous  growth. 

2.  Sexual  reproduction  normally  implies  (a)  special  reproductive  cells, 
distinct  from  the  body  ;  (/-')  the  dimorphism  of  these  cells ;  [c)  their 
physiological  dependence, — the  ovum  being  unproductive  without  the  sjier- 
matozoon,  and  vice  versa. 

3.  The  discoveries  of  Camerarius,  Amici,  Kolreuter,  Sprengel,  and 
others,  laid  the  foundations  of  our  knowledge  of  sexual  reproduction  in 
plants. 

4.  The  history  of  research  on  fertilisation  in  animals  well  illustrates  the 
gradually  increasing  precision  of  scientific  inquiry. 

5.  The  conjugation  processes  seen  in  Protozoa  are  of  much  importance 
in  suggesting  the  origin  of  differentiated  fertilisation. 

6.  The  origin  of  fertilisation  may  be  traced  through  the  following 
grades  : — {a)  plasmodial  union,  (/>)  multiple  conjugation,  {i)  ordinary  con- 
jugation, {(/)  union  of  dimorphic  cells,  {c)  fertilisation  of  ovum  by  sperma- 
tozoon. 

7.  I>oth  in  plants  and  animals  hybridisation  is  often  successful,  but  the 
offspring  frequently  tend  to  be  sterile.  This,  however,  must  not  be 
exaggerated. 

LITERATURE. 

See  the  already  noted  works  of  Balfour,  Van  IJeneden,  Carnoy,  Geddes, 
Haddon,  Ilensen,  Ilertwig,  M'Kendrick,  Sachs,  and  Vines. 

?'or  recent  papers  see  Boveri,  Th.,  Zellen  Studicn  ;  Jenaische  Zf.-itschrift 
fiir  Naturwissenschaften,  1887-88;  Zoological  Record,  from  1886;  antl 
Journal  of  Royal  Microscopical  Society. 


CHAPTER  XII. 

Theory  of  Fertilisation. 

In  his  49th  Exercitation  on  the  "  efficient  cause  of  the  chicken," 
Harvey  thus  quaintly  expresses  what  has  ahva)'S  been,  and  still 
is,  a  baffling  problem  : — "  Although  it  be  a  known  thing  sub- 
scribed by  all,  that  the  foetus  assumes  its  original  and  birth 
from  the  male  and  female,  and  consequently  that  the  egge  is 
produced  by  the  cock  and  henne,  and  the  chicken  out  of  the 
egge,  yet  neither  the  schools  of  physicians  nor  Aristotle's  dis- 
cerning brain  have  disclosed  the  manner  how  the  cock  and  its 
seed  doth  mint  and  coine  the  chicken  out  of  the  egge."  The 
old  theories  on  the  subject  are  more  curious  than  profitable,  a 
fact  not  to  be  wondered  at  since  it  is  really  only  within  the 
last  fifty  years  that  the  fundamental  fact  of  the  union  of  the 
sex-cells  has  been  observed. 

§  I.  Old  Theories  of  Fertilisation. — (^7)  From  Pythagoras 
and  Aristotle  on  to  the  "Ovists,"  of  whom  we  have  already 
spoken  (p.  84  ),  numerous  naturalists  have  held  the  opinion 
that  the  ovum  was  the  all-important  element,  which  only 
required  to  be  awakened  to  development  by  contact  with  the 
male  fluid  or  male  elements.  It  must  be  allowed,  that  while 
ova  may  exceptionally  develop  without  sperms,  the  latter  never 
come  to  anything  apart  from  ova.  This  will  be  less  insisted  on, 
however,  when  it  is  recognised  that  in  reality  the  ovum  is  not  so 
fiiirly  comparable  with  the  spermatozoon  as  with  the  mother- 
sperm-cell.  It  must  be  allowed,  too,  that  there  is  much  to 
warrant  us  in  thinking  of  the  sperm  as  an  element  which 
stimulates  the  ovum  to  division  ;  yet  this  will  be  recognised  as 
only  approximate  language,  when  the  facts  of  the  intimate 
nuclear  union  are  fully  appreciated. 

{li)  In  contrast  to  the  above  opinion,  we  find  ingenious 
thinkers,  so  widely  separate  in  time  as  Democritus  and  Paracelsus, 
regardiiig  the  male  fluid  as  very  important,  forestalling  Buff'on 
and  Darwin  in  fact  in  considering  it  in  a  sense  an  extract  or 


158  THE    EVOLUTION    UF    SEX. 

concentrated  essence  of  the  whole  body.  But  it  was  only  after 
the  spermatozoa  were  themselves  detected  that  their  importance 
became  unduly  exaggerated,  in  the  minds  of  those  who  seem 
almost  to  have  been  nicknamed  "animalculists."  It  seems 
probable  enough  that  Leeuwenhoek  himself  (1677)  saw  the 
spennatozoon  entering  the  ovum, — he  at  least  said  that  he  did, 
— but  that  did  not  prevent  him  from  ascribing  to  the  male 
elements  all  the  credit  of  development.  This  became,  as  we 
have  seen,  a  favourite  hypothesis,  and  imagination  supplied 
more  than  modern  magnifiers  to  those  observers  who  detected 
in  the  spermatozoon  the  members  and  lineaments  of  the 
future  organism.  After  this  the  discovery  that  the  sperm 
supplies  half  the  nucleus  of  the  fertilised  ovum,  and  half  the 
nuclei  of  the  two  first  daughter-cells,  seems  almost  a  little  thing. 
The  polemic  of  modern  science  has  this  advantage  at  least,  that 
when  two  competent  authorities  on  the  same  subject  assert  the 
same  thing,  we  may  generally  believe  them, 

(c)  The  third  opinion,  that  both  elements  are  of  essential 
and  inseparable  import,  is  obviously  alone  consistent  with  the 
facts.  This  view  also  has  had  its  gradual  development,  only 
one  phase  of  which  need  be  noticed.  Even  after  the  nature  of 
the  spermatozoa  as  male-cells  was  recognised,  that  is  to  say,  even 
within  the  last  fifty  years,  an  old  conception  of  the  male  in- 
fluence lingered  persistently.  This  namely,  that  contact  was 
not  essential,  but  that  a  "sort  of  contagion,"  a  "breath  or 
miasma,"  "a  plastical  vertue,"  "without  touching  at  all,  unless 
through  the  sides  of  many  mediums,"  was  sufficient  to  effect 
what  we  call  fertilisation.  The  above  expressions  are  used  by 
Harvey,  who  further  says,  "  this  is  agreed  upon  by  universal 
consent,  that  all  animals  whatever,  which  arise  from  male  and 
female,  are  generated  by  the  coition  of  both  sexes,  and  so 
begotten  as  it  w^re  pei'  contaij^iuiii  alicjuod.''''  L)e  Graaf  attempted 
in  vain  to  give  more  precision  to  this  "contagion"  in  his  theory 
of  an  '■'■  au?'a  seminalis,^^  or  seminal  breath  which  passed  from 
the  male  fluid  to  the  ovum.  But  the  conception  of  an  "aura  " 
was  only  a  verbal  cloak  for  that  absence  of  definite  knowledge 
which  the  slow  progress  of  observation  still  necessitated.  The 
theory  was  partly  strengthened  by  a  number  of  erroneous  obser- 
vations, which  seemed  to  show  that  successful  fertilisation  could 
occur  when  the  genital  passages  of  the  female  were  apparently 
blocked  by  malformation  or  disease.  Spallanzani  gave  a  death- 
blow  to   the   theory  of  an   "aura,"  by  showing  ex})erimentally 


THEORY    OF    FERTILISAIION.  159 

that  contact  of  the  male  fluid  with  the  ovum  was  absolutely 
necessary.  Even  he,  however,  went  away  h'om  the  true  con- 
clusion, by  maintaining  that  the  fertile  male  fluid  of  toads  was 
destitute  of  spermatozoa.  That  the  above  vague  conceptions 
have  been  replaced  by  the  certain  conclusion,  that  intimate 
cellular  union  is  the  sine  qua  nou  of  fertilisation,  we  have  already 
emphasised. 

i:^  2.  Modern  T/ieo7'ies  of  Fertilisation  —  Morphological. — 
Recent  investigators  of  the  facts  of  fertilisation  have  generalised 
their  results  in  different  ways  according  to  their  dominant  bias. 
Some  mainly  restrict  themselves  to  stating  the  morphological 
facts,  and  to  emphasising  the  relative  importance  of  cell-sub- 
stance and  of  nuclei  in  the  union  ;  others  attack  the  deeper 
problem  of  the  physiological  import  of  the  process, — a  problem 
the  full  solution  of  which  is  still  remote  ;  while  others  have 
confined  themselves  rather  to  discussing  the  uses  of  fertilisation 
in  relation  to  the  species.  Some  representative  positions  on 
each  of  these  planes  must  be  sketched  ;  and,  first  of  all,  the  more 
morphological  theories,  and  the  very  important  question  whether 
the  union  of  nuclei  is  everything,  or  whether  the  union  of  cell- 
substance  has  also  its  import. 

{a)  Herhoig's  Vie7v. — Professor  O.  Hertwig,  who  was  one  of  the  first 
carefully  to  follow  out  the  details  of  fertilisation  in  animals,  thus  sums  up  his 
"  TJieorie  der  BefnicJifnng"  : — "In  fertilisation,  distinctly  demonstrable 
morphological  processes  occur.  Of  these  the  important  and  essential  one 
is  the  union  of  two  sexually  differentiated  cell-nuclei,  the  female  nucleus  of 
the  ovum  and  the  male  nucleus  of  the  sperm.  These  contain  the  fer- 
tilising nuclear  substance,  which  is  an  organised  substance,  and  acts  as  such 
in  the  process.  The  female  nuclear  substance  transmits  the  characters  of 
the  mother,  the  male  nucleus  those  of  the  father,  to  the  offspring."  The 
nucleus  is  thus  the  essential  element  both  in  fertilisation  and  in  inheritance. 
{h)  Slrashurge7-''s  Vieiv.  —  What  Hertwig  maintains  for  animals, 
Strasburger  does  for  plants.  "The  process  of  fertilisation  depends  upon 
the  union  of  the  sperm  nucleus  with  the  nucleus  of  the  egg-cell  ;  the  cell- 
substance  (cytoplasm)  does  not  share  in  the  process."  "The  cell-sub- 
stance of  the  pollen-grain  is  only  the  vehicle  to  conduct  the  generative- 
nucleus  to  its  destination."  It  may  become  nutritive,  he  allows  however, 
to  the  germ-rudiment.  "  Generally  the  uniting  nuclei  are  almost  perfectly 
alike,"  though  there  may  be  slight  differences  in  the  size  of  the  nucleoli. 
"The  two  cell-nuclei  do  not  differ  in  their  nature,  they  are  not  sexually 
differentiated  in  the  ways  that  the  individuals  are  from  which  they  originate. 
All  sex-differentiations  only  serve  to  bring  together  the  two  nuclei  essential 
to  the  sexual  process." 

The  opinions  of  these  two  authorities  are  certainly  representative,  and 
they  both  agree  in  emphasising  that  the  nuclei  are  all-important,  and  that  it 
does  not  matter  much  about  the  union  of  cell-substance.  Some  objections 
to  this  view  must  be  noticed,     {a)  It  is  permissible  to  doubt  whether  the 


l6o  'J'HE    EVOLUTION    OF    SEX. 

recent  concentration  of  attention  upon  the  nucleus  has  not  led  to  some 
under-apj)i'eciation  of  the  general  protoplasm.  In  the  permanent  conjuga- 
tion of  two  cells,  the  entire  contents  of  the  two  cells  are  obviously  fused  ; 
and  even  when  the  union  is  temporary,  Joseph  has  observed  what  looks 
like  an  interchange  of  protoplasmic  as  well  as  of  nuclear  substance. 
(/')  There  are  a  few  observers  still,  such  as  Nussbaura,  who  maintain 
that  in  fertilisation  in  animals  the  substance  of  the  sperm  is  important 
as  well  as  its  nucleus.  {c)  Strasburger  notes  the  minimal  quantity  of 
cell-substance  so  often  present  round  the  male  nucleus,  and  urges  that 
if  it  were  important  there  would  surely  be  more  of  it.  But  it  is  quite 
conceivable  that  a  minimal  quantity  of  highly  active  protoplasm  might 
have,  like  a  ferment,  a  momentous  influence  on  a  large  quantity  of  a 
different  character.  (</)  The  researches  of  Boveri  show,  that  though  the 
union  of  nuclei  is  so  essential,  the  protoplasmic  activity  and  share  in  the 
process  are  also  considerable.  It  appears  to  us  a  fact  well  worthy  of  con- 
sideration, that  according  to  this  authority  the  sperm  brings  with  it  into 
the  ovum  a  protoplasmic  centre — a  "  centrosoma" — which  appears  to  be  of 
much  importance  in  the  preparation  for  division.  In  this  preparation, 
according  to  Boveri,  the  "  muscular  fibrils"  of  a  special  kind  of  protoplasm 
(or  archoplasma)  literally  move  the  nuclear  elements.  "The  movement 
of  the  elements  is  wholly  the  result  of  the  contraction  of  the  attached 
fibrils,  and  the  final  arrangement  of  these  nuclear  elements  in  the  'equa- 
torial plate'  is  the  result  of  the  action  of  the  archoplasmic  sphere  exerted 
through  the  fibrils."  Now  this  specially  active  protoplasm,  which  the 
skilful  observer  seems  to  have  succeeded  in  fixing,  has  its  centre.  There 
are  two  central  corpuscles,  each  "  ruling  a  sphere  of  archoplasma."  Where 
then  do  these  centres  come  from  ?  "  It  is  probable,"  Boveri  says,  "  that 
the  spermatozoon  brings  a  centrosoma  into  the  ovum,  and  that  this  by 
division  forms  two  centres.  Since  these  two  corpuscles  condition  the 
division,  the  dependence  of  this  upon  the  presence  of  the  spermatozoon 
is  for  the  Ascaris  ovum  explained."  We  have  given  these  details,  technical 
as  they  are,  because  they  seem  to  us  to  show  clearly  that  it  is  rash  to 
deny  that  even  the  minimal  cell-substance  of  the  spermatozoon  may,  as 
well  as  its  nucleus,  have  a  momentous  influence  in  fertilisation. 

§  3.  Physiological  Theories  of  Fe7'lilisation.  — -  The  mor- 
phological facts,  established  and  verifiable  by  observation,  form 
the  basis  from  which  to  attack  the  deeper  problem  of  the 
physiology  of  fertilisation.  Here  experiment  is  almost  insuper- 
ably difficult ;  only  a  few  incidental  results  are  as  yet  available  ; 
the  suggestions  thrown  out  by  various  naturalists  must  therefore 
be  appreciated  according  to  their  consistence  with  the  general 
l)rinciples  of  physiology,  and  with  the  general  theory  of  sex 
and  reproduction.  To  some  they  may  still  appear  a  page  of 
probabilities. 

Sachs  compares  the  action  of  the  male  element  upon  the 
egg-cell  to  that  of  a  ferment.  De  Bary  also  suggests  that  pro- 
found chemical  differences  exist  between  the  two  elements. 
Very  suggestive  is  the  view  of  Rolph,  who  regarded  the  process 


THEORY    OF    FERTILISATION.  l6l 

as  essentially  one  of  mutual  digestion.     His  vivid  words  well 
deserve  quotation  : — 

"  Conjugation  occurs  when  nutrition  is  diminished,  whether  this  be  due 
to  want  of  light,  or  to  the  lowered  temperature  of  autumn  and  winter,  or 
to  a  reduction  of  the  organisms  to  mimimal  size.  It  is  a  necessity  for 
satisfaction,  a  gnawing  hunger,  which  drives  the  animal  to  engulf  its 
neighbour,  to  'isophagy. '  The  process  of  conjugation  is  only  a  special 
form  of  nutrition,  which  occurs  on  a  reduction  of  the  nutritive  income,  or 
an  increase  of  the  nutritive  needs,  in  consequence  of  the  above-mentioned 
conditions.  It  is  an  'isophagy,'  which  occurs  in  place  of  'heterophagy. ' 
The  less  nutritive,  and  therefore  smaller,  hungrier,  and  more  mobile 
organism  we  call  the  male, — the  more  nutritive  and  usually  relatively  more 
quiescent  organism,  the  female.  Therefore  too  is  it,  that  the  small  starv- 
ing male  seeks  out  the  large  well-nourished  female  for  purposes  of  conjuga- 
tion, to  which  the  latter,  the  larger  and  better  nourished  it  is,  is  on  its 
own  motive  less  inclined."  Cienkowski  has  also  inclined  to  a  similar  view, 
regarding  conjugation  as  equivalent  to  rapid  assimilation. 

Simon  also  seeks  to  establish  the  following  among  other  vague  con- 
clusions : — Sexuality  has,  he  says,  arisen  twice  (we  should  say  much  oftener), 
once  among  plants,  again  among  Protozoa.  Two  similar  cells  unite  ' '  in  order 
to  reach  the  limit  of  their  individuality."  In  both  kingdoms  the  union  is 
at  first  protective,  though  in  a  different  fashion  in  the  two  cases.  In  the 
progressive  differentiation,  these  two  sex-cells  are  usually  so  constructed 
that  the  loss  of  substance  in  the  union  is  reduced  to  a  minimum,  hence  the 
small  mobile  male  and  the  large  quiescent  female  cells.  The  union  brings 
about  a  chemico-physical  process,  which  makes  the  female  cell  capable  of 
independent  nutrition  and  growth,  and  evokes  potential  properties  into 
actual  life. 

In  marked  contrast  to  Rolph's  suggestion,  and  the  view  of  all 
those  who  believe  that  the  sex-cells  are  profoundly  different,  is 
the  opinion  maintained  by  Weismann.  He  denies  that  there 
is  a  dynamical  action  in  fertilisation.  The  momentous  effect  is 
merely  the  sudden  doubling  of  the  mass  of  the  nucleus.  "  The 
physiological  values  of  sperm  and  egg-cell  are  equal ;  they  are 
as  I  :  I.  We  can  hardly  ascribe  to  the  body  of  the  ovum 
a  higher  import  than  that  of  being  the  common  nutritive  basis 
for  the  two  conjugating  nuclei."  The  external  differences  which 
are  so  obvious  are  only  important  as  means  towards  the  con- 
jugation of  similar  nuclei.  ''The  germ-plasma  in  the  male  and 
female  reproductive  cells  is  identical."  Previous  to  the  essential 
moment  of  fertilisation,  half  of  the  germ-plasma  is  given  off 
from  the  germinal  vesicle  of  the  ovum  in  forming  the  second 
polar  body.  Development  will  not  take  place  unless  the  loss 
be  made  good,  and  the  original  mass  restored.  This  is  what 
the  sperm  does  in  fertilisation.  In  short,  to  Weismann  the 
process  is  quantitative  rather  than  qualitative. 

L 


1 62  THE    EVOLUTION    OF    SEX. 

This  supposition  appears  to  us  to  be  open  to  criticism,  (i.)  That  the 
nuclei  are  alone  important  in  fertilisation,  and  that  the  cell  substance  is  a 
mere  adjunct,  cannot  be  said  to  be  proved,  and  we  have  already  noted  some 
of  the  facts  which  tell  the  other  way.  (2.)  The  structure  of  a  cell  is 
recognised  by  all  to  be  an  expression  of  its  dominant  protoplasmic  pro- 
cesses. The  sex-cells  are  usually  highly  dimorphic,  and  even  Strasburger 
allows  that  there  may  be  minor  differences  in  their  nuclei,  as  well  as  the 
marked  divergence  in  their  cell-substance.  The  nucleus  cannot  be 
regarded  as  an  isolated  element,  but  as  one  which  shares  in  the  general 
life  of  the  cell.  We  have  already  interpreted  the  differentiated  male  and 
female  cells  as  respectively  katabolic  and  anabolic,  and  see  no  reason  for 
doubting,  in  spite  of  structural  resemblance  in  the  rough  features  of  nuclei 
(all  that  we  know),  that  this  difference  saturates  through  the  elements.  (3.) 
If  the  only  important  matter  be  the  quantitative  restoration  of  the  original 
amount  of  germ-plasma  in  the  female  nucleus,  it  seems  difficult  to  under- 
stand the  phenomena  of  conjugation,  whether  permanent  or  transitory,  from 
which  we  believe  fertilisation  to  have  originated.  (4. )  That  the  normal  ovum 
should  lose  half  its  quantity  of  germ-plasma,  only  to  regain  a  similar  quantity 
in  fertilisation,  certainly  appears  a  curiously  circuitous  process.  (5.)  The 
occasional  possibility  of  inducing  division  by  replacing  the  sperms  with 
other  stimuli,  seems  to  point  to  a  dynamical  or  chemical  action,  which 
Weismann  denies. 

We  are  bound,  of  course,  to  admit  the  importance  of  the  established 
facts  of  nuclear  union,  and  agree  with  Boveri,  that  the  complexity  of  the 
morphological  facts  shows  the  present  impossibility  of  supposing  that  they 
can  be  fully  expressed  in  chemical  terms.  But  a  due  impression  of  the 
marvellous  "individuality  "  of  the  nuclear  elements  may  be  combined  with 
a  general  physiological  interpretation  of  the  entire  process. 

It  has  been  already  noted,  in  regard  to  the  origin  of  fertihsa- 
tion,  that  the  ahnost  mechanical  flowing  together  of  exhausted 
cells  is  connected  by  the  stages  of  multiple  conjugation  with 
the  ordinary  form  of  the  latter,  while  the  respective  differentia- 
tion of  the  two  elements  effects  the  transition  to  fertilisation 
proper.  Historically,  then,  fertilisation  is  comparable  to  mutual 
digestion,  and,  though  bound  up  with  reproduction,  has  arisen 
from  a  nutritive  want.  With  the  differentiation  of  the  elements 
on  anabolic  and  katabolic  lines,  the  nature  of  the  fertilising  act 
becomes  more  definite.  The  essentially  katabolic  male  cell, 
getting  rid  of  all  accessory  nutritive  material  contained  in  the 
sperm-cap  and  the  like,  brings  to  the  ovum  a  supply  of 
characteristic  waste  products  or  katastates,  which  stimulate  the 
latter  to  division.  The  profound  chemical  differences,  surmised 
by  some,  are  intelligible  as  the  outcome  of  the  predominant 
anabolism  and  katabolism  in  the  two  elements.  I'hc  union  of 
the  two  sets  of  products  restores  the  normal  balance  and 
rhythm  of  cellular  life.  Rolph's  suggestion  is  thus  included 
and  defined. 


THEORY    OF    FERTILISATION.  1 63 

§  4.  Uses  of  Fer tills atio7i  to  the  Species. — Not  a  few  natu- 
ralists have  passed  from  the  individual  aspect  of  fertilisation  to 
its  general  import  in  relation  to  the  life  of  the  species.  Why 
should  fertilisation  occur  at  all,  if  parthenogenesis  in  some  cases 
works  so  well  ?  Part  of  this  question  is  almost  illegitimate,  if 
the  existence  of  male  and  female  be,  as  we  think,  simply  the 
expression  of  a  more  developed  swing  of  "  the  organic  see- 
saw "  between  anabolism  and  katabolism.  The  answers  have, 
however,  much  interest,  and  are  valuable,  so  long  as  they  are 
not  magnified  so  as  to  hide  the  deeper  physiological  problems 
lying  below.  The  origin  and  physiological  import  of  fertilisa- 
tion can  never  be  explained  by  any  elucidation  of  its  subsequent 
advantageousness. 

The  two  naturalists  who  have  recently  reached  the  most 
valuable  results  in  regard  to  the  uses  of  fertilisation  are  Maupas 
and  Weismann.  This  they  have  done  by  very  different  paths, — 
Maupas,  in  working  out  the  details  of  conjugation  in  infusorians  ; 
Weismann,  in  his  wider  studies  on  the  problems  of  heredity  and 
evolution.  To  Maupas,  fertilisation  is  necessary  to  prevent  the 
death  of  the  species ;  to  Weismann,  fertilisation  is  the  ever- 
recurrent  beginning  of  new  vital  changes,  and  the  continual 
preservation  at  the  same  time  of  the  relative  constancy  of  the 
species.  Several  naturalists  of  the  highest  reputation  have 
regarded  fertilisation  as  a  process  which  supplied  a  fresh  life- 
impulse  to  the  species.  Thus  Galton  has  insisted,  with  much 
clearness  and  force,  on  the  liability  of  asexual,  or  what  he  calls 
unisexual  multiplication  to  end  in  degeneration  or  extinction, 
and  on  the  necessity  of  double  parentage  for  the  preservation 
and  progress  of  the  species.  Similarly,  Van  Beneden,  Biitschli, 
and  Hensen  have  all  spoken  of  the  process  as  a  rejuvenescence 
{rejeunissement,  Verjiingung).  The  asexual  process  of  cell- 
multiplication  is  limited ;  conjugation  in  lower,  fertilisation  in 
higher  organisms  supply  the  recurrent  impulse  which  keeps  the 
life  of  the  species  young.  According  to  Van  Beneden, — "  The 
faculty  which  cells  possess  of  multiplying  by  division  is  limited. 
There  comes  a  time  when  they  can  divide  no  further,  unless 
they  undergo  rejuvenescence  by  fertilisation.  In  animals  and 
plants,  the  only  cells  capable  of  being  rejuvenesced  are  the 
eggs ;  the  only  cells  capable  of  rejuvenescing  these  are  the 
sperms.  All  the  other  parts  of  the  individual  are  devoted  to 
death.  Fertilisation  is  the  condition  of  the  continuity  of  life. 
Par  elle  le  generateur  echappe  a  la  mort."     Hensen,  in  his 


164  THE    EVOLUTION    OF    SEX. 

admirable  "Physiology  of  Reproduction,"  expresses  the  same 
when  he  says  : — "  By  normal  fertilisation,  death  is  warded  off 
(ferngehalten)  from  the  germ  and  its  products."  Biitschli  has 
interpreted  conjugation  in  similar  terms. 

Weismann  quotes  the  three  opinions  just  mentioned,  and 
vigorously  criticises  them.  He  demands  evidence  for  the 
limitation  of  asexual  reproduction  assumed  above,  and  speaks 
of  the  "  impossibility  of  proof."  The  whole  "  conception  of 
rejuvenescence  has  something  indefinite  and  misty  about  it." 
(Some  may  be  obliged  to  plead  guilty  to  a  similar  impression  in 
regard  to  Weismann's  Keimplasma.)  "  How  can  one  think  that 
an  infusorian,  which  by  continued  division  has  at  length 
exhausted  its  reproductive  capacity,  will  regain  the  same  by 
uniting  and  fusing  with  another  which  has  also  lost  its  power  of 
further  division  ?  Twice  nothing  cannot  give  one ;  or  if  one 
assumes  that  in  each  animal  there  persists  only  half  the  repro- 
ductive capacity,  so  that  the  two  together  would  form  one,  this 
one  can  hardly  call  '  rejuvenescence.'  It  would  be  simply  an 
addition,  as  is  under  other  circumstances  attained  by  simple 
growth, — that  is,  if  we  leave  out  of  account  what  in  my  eyes  is 
the  most  important  moment  in  conjugation,  viz.,  the  mingling 
of  two  heredity-tendencies  {^Vererbimgstetidenzeii).''''  (Does  Pro- 
fessor Weismann  not  feel  that  there  is  something  "  indefinite 
and  misty"  even  about  this?)  He  sarcastically  compares  the 
two  exhausted  individuals  to  two  exhausted  rockets,  which  are 
supposed  to  rejuvenesce  in  mutually  affording  the  constituents 
of  nitroglycerine.  More  forcibly  he  urges  the  difficulty  sug- 
gested by  continued  parthenogenesis, — a  difficulty  which  we  shall 
afterwards  have  to  discuss.  "  To  the  conception  of  rejuvenes- 
cence," he  says,  in  conclusion,  "  I  could  only  agree,  if  it  were 
proved  that  multiplication  by  division  can  never, — not  merely  in 
certain  conditions, — but  never  continue  unlimitedly.  This 
cannot,  however,  be  proved,  just  as  little  as  the  reverse."  But 
Weismann  must  surely  admit,  that  the  demonstration  of  even 
some  cases  where  species,  normally  reproducing  asexually,  come 
to  an  absolute  standstill  if  conjugation  be  prevented,  would 
give  considerable  strength  to  the  interpretation  of  fertilisation 
as  rejuvenescence.  Such  cases  have,  happily,  come  to  hand,  as 
we  shall  now  see. 

We  have  already  referred  to  Maupas's  proof  of  true  sexual 
union  in  ciliated  infusorians.  By  an  elaborate  process  of 
nuclear   division,    disruption,    elimination,  interchange,    union, 


THEORY    OF    FERTILISATION.  165 

and   reconstruction,  two    "slipper    animalcules"  fertilise  one 
another.     What  is  the  meaning  of  all  this? 

Each  infusorian,  after  conjugation,  proceeds  to  divide,  but 
the  results  are  to  all  appearance  the  same  as  it  previously  pro- 
duced.    There  is  no  special  sexually  produced  generation. 

It  has  been  often  alleged  that  the  subsequent  dividing  is 
accelerated  by  conjugation ;  but  Maupas  finds  that  this  is  not 
so.  The  reverse  in  fact  is  true, — it  is  a  loss  of  time.  While  a 
pair  of  infusorians  [Onychodromus  grandis)  were  indulging  in  a 
single  conjugation,  another  had  become,  by  ordinary  asexual 
division,  the  ancestor  of  from  forty  thousand  to  fifty  thousand 
individuals. 

Moreover,  the  intense  internal  change  preparatory  to  fer- 
tilisation, and  the  general  inertia  during  subsequent  reconstruc- 
tion, not  only  involve  loss  of  time,  but  expose  the  infusorians 
to  great  risk.  It  seems  then  like  a  condition  of  danger  and  death 
rather  than  of  multiplication  and  birth. 

The  riddle  was,  in  part  at  least,  solved  by  a  long  series  of 
careful  observations.  In  November  1885,  M.  Maupas  isolated 
an  infusorian  {Stylonichia  picstulata)^  and  observed  its  genera- 
tions till  March  1886.  By  that  time  there  had  been  two 
hundred  and  fifteen  generations  produced  by  ordinary  division, 
and  since  these  lowly  organisms  do  not  conjugate  with  near 
relatives,  there  had,  of  course,  been  no  sexual  union. 

What  was  the  result  ?  At  the  date  referred  to,  the  family 
was  observed  to  have  exhausted  himself.  They  were  not  old 
exactly,  but  they  were  being  born  old.  The  asexual  division 
came  to  a  standstill,  and  the  powers  of  nutrition  were  also  lost. 

Meanwhile,  however,  several  of  the  individuals,  before  the 
generations  had  exhausted  themselves,  had  been  removed  to 
another  basin,  where  they  conjugated  with  unrelated  forms  of 
the  same  species.  One  of  these  was  again  isolated,  and  watched 
for  five  months.  The  usual  richness  of  successive  generations 
occurred ;  members  removed  at  different  stages  were  again 
observed  to  conjugate  successfully  with  unrelated  forms,  and 
this  was  done  on  to  the  one  hundred  and  thirtieth  generation. 
After  that,  however,  the  family  being  again  near  its  end,  the 
removal  was  no  longer  any  use.  About  the  one  hundred  and 
eightieth  generation,  the  strange  sight  was  seen  of  individuals 
of  the  same  family  attempting  to  unite  with  one  another.  The 
results  were,  however,  7iil,  and  the  conjugates  did  not  even 
recover  from  the  effects  of  their  forlorn  hope. 


1 66  THE    EVOLUTION    OF    SEX. 

Without  the  normal  sexual  union,  then,  the  family  becomes 
senile.  Powers  of  nutrition,  division,  and  conjugation  with 
unrelated  forms,  come  to  a  standstill.  This  senile  degeneration 
is  very  interesting.  The  first  symptom  is  decrease  in  size, 
which  may  go  on  till  the  individuals  only  measure  a  quarter  of 
their  normal  proportions.  Various  internal  structures  then 
follow  suit,  "until  at  last  we  get  formless  abortions,  incapable  of 
living  and  reproducing  themselves."  The  nuclear  changes  are 
no  less  momentous.  The  important  para-  or  micro -nucleus 
may  partially  or  completely  atrophy,  and  conjugation  is  thus 
fatally  sterile.  The  larger  nucleus  may  also  become  affected, 
"  the  chromatin  gradually  disappearing  altogether."  Physiolo- 
gically too,  the  organisms  become  manifestly  weaker,  though 
there  is  what  the  author  calls  a  "  surexcitation  sexuelle."  Such 
senile  decay  of  the  individuals  and  of  the  isolated  family  in- 
evitably ends  in  death. 

The  general  result  is  evident.  Sexual  union  in  those  infus- 
orians,  dangerous  perhaps  for  the  individual  life, — a  loss  of  time 
so  far  as  immediate  multiplication  is  concerned, — is  in  a  new 
sense  necessary  for  the  species.  The  life  runs  in  cycles  of 
asexual  division,  which  are  strictly  limited.  Conjugation  with 
unrelated  forms  must  occur,  else  the  whole  life  ebbs.  ^Vithout 
it,  the  Protozoa,  which  some  have  called  "  immortal,"  die  a 
natural  death.  Conjugation  is  the  necessary  condition  of  their 
eternal  youth  and  immortality.  Even  at  this  low  level,  only 
through  the  fire  of  love  can  the  phoenix  of  the  species  renew 
its  youth. 

At  the  beginning  of  this  century,  the  too-much-forgotten 
biologist  Treviranus  directed  attention  to  fertilisation  as  a 
source  of  variation,  and  his  suggestion  has  been  several  times 
independently  revised. 

Thus  Brooks,  to  whose  works  we  have  repeatedly  referred, 
has  emphasised  not  only  the  importance  of  fertilisation  as  a  source 
of  progressive  change,  but  further,  that  the  male  element  is 
much  the  more  important  in  this  connection. 

Similarly,  though  on  somewhat  different  lines,  Weismann 
finds  in  the  mingling  of  male  and  female  Keimplasmas  the 
source  of  those  variations  on  which  natural  selection  operates. 
Rejecting  as  he  does  the  alleged  inheritance  of  acquired 
characters,  he  finds  the  fountain  of  change  in  sexual  repro- 
duction. "  Sexual  reproduction  is  well  known  to  consist  in  the 
fusion  of  two  contrasted  reproductive  cells,  or  perhaps  even  in  the 


THEORY    OF    FERTILISATION.  iC'J 

fusion  of  their  nuclei  alone.  These  reproductive  cells  contain 
the  germinal  material  or  Keimplasma,  and  this  again,  in  its 
specific  molecular  structure,  is  the  bearer  of  the  hereditary 
tendencies  of  the  organisms  from  which  the  reproductive  cells 
originate.  Thus  in  sexual  reproduction,  two  hereditary  tend- 
encies are  in  a  sense  intermingled.  In  this  mingling,  I  see  the 
cause  of  the  hereditary  individual  characteristics ;  and  in  the 
production  of  these  characters,  the  task  of  sexual  reproduction. 
It  has  to  supply  the  material  for  the  individual  differences  from 
which  selection  produces  new  species." 

But  this  very  reasonable  contention  hardly  appears  to 
consist  with  Weismann's  quantitative  interpretation  of  the 
process  of  fertilisation.  Nor  is  it  evident  how  the  diversities 
of  the  male  and  female  plasmas  became  such  as  their  results 
indicate  them  to  be,  if  Weismann  be  correct  in  maintaining 
that  no  modifications  of  the  body  influence  the  reproductive 
elements. 

Brooks  and  Weismann  have  at  any  rate  maintained  a  thesis 
which  few  will  be  inclined  to  oppose,  that  sexual  union  is 
productive  of  variation.  To  discuss  the  relations  of  this 
view  to  other  theories  of  variation  is  not  here  relevant, 
nor  can  we  do  more  than  mention  the  reasonable  sugges- 
tion of  Hatschek,  that  sexual  reproduction  is  a  remedy  against 
the  operation  of  injurious  variations.  For  we  can  readily 
imagine,  that  the  excess  of  some  particular  line  of  anabolic  or 
katabolic  differentiation  may  be  neutralised  through  fertilisation. 
In  this  way  one  is  led  to  speculate,  whether  the  constant  pairing 
of  diseased  individuals  may  not  sometimes  be  more  mercifully 
condoned  by  nature  than  we  have  been  accustomed  to  think. 


1 68  THE    EVOLUTION    OF    SEX. 


SUMMARY. 

1.  Old  theories  of  "ovists,"  "  animalculists,"  and  of  the  "aura 
seminalis." 

2.  Modern  morphological  theories  incline  to  lay  the  whole  emphasis 
upon  the  nuclei.  The  conclusions  of  Hertwig  and  Strasburger  are  strongly 
in  favour  of  this  view.  The  claims  of  the  cell-substance  and  general  proto- 
plasm must  not,  however,  be  overlooked.  Many  facts,  such  as  those  demon- 
stiated  by  Boveri,  show  that  the  protoplasm  is  also  important. 

3.  Modern  physiological  theories  of  fertilisation  are  necessarily  very 
tentative.  Sachs  compares  it  to  fermentation  ;  Rolph  to  mutual  digestion. 
To  Weismann,  the  process  appears  quantitative  rather  than  qualitative,  so 
far  as  the  subsequent  division  is  concerned.  Half  of  the  Keimplasma  which 
the  ovum  has  surrendered  with  the  second  polar  globule  is  restored  by  the 
sperm-nucleus.  The  two  nuclei  are  alike,  and  thus  there  is  virtually  no  sex. 
Protest  against  this.  The  male  cell  brings  to  the  ovum  a  supply  of  char- 
acteristic katastates. 

4.  Uses  of  fertilisation  to  the  species.  Many  regard  fertilisation  as  a 
necessary  rejuvenescence  of  the  life  of  the  species.  Weismann  criticises 
this  view,  but  his  criticism  must  be  read  in  the  light  of  the  researches  of 
Maupas,  who  has  shown  that  without  conjugation  the  members  of  an 
isolated  family  of  infusorians  eventually  cease  to  feed  and  divide,  passing 
through  stages  of  degeneration  and  senility  to  extinction.  In  this  case, 
conjugation  is  essential  to  the  continued  vitality  of  the  species.  According 
to  Brooks,  fertilisation  is  an  important  source  of  variation  ;  according  to 
Weismann,  it  is  really  the  sole  source. 

LITERATURE. 

Works  already  cited. 

Hertwig,  O. — Das  Problem  der  Befruchtung,  &c. ;  Jenaische  Zeitschrift 

fiir  Naturwissenschaften,  XVHI.,  1S85. 
Maupas,  E. — Comptes  Rendus,    1886,    1887  ;  and  Archives  de  Zoologie 

experimentale,  1888. 
Strasburger,  E. — Neue  Untersuchungen  iiber  den  Befruchtungsvorgang 

bei  den  Phanerogamen,  als  Grundlage  fiir  eine  Theorie  der  Zeugung. 

Jena,  1884. 
Weismann,  A.  —  C//.  «V., especially  Die  Bedeutung der  sexuellen  Fortpflan- 

zung  fiir  die  Selektions-Theorie.     Jena,  i< 


CHAPTER  XIII. 

Degenerate  Sexual  Reproduction  or  Parthenogenesis. 

§  I.  History  of  Discovery. — From  very  early  times  there  appears 
to  have  been  an  impression,  that  in  exceptional  circumstances 
reproduction  might  occur  without  fertilisation.  Even  Aristotle 
gave  reasons  for  believing  that,  without  sexual  union,  the  un- 
fertilised eggs  of  the  honey-bee  might  give  rise  to  perfect  adults. 
We  now  know  that  he  was  right,  in  his  conclusion  at  least,  so 
far  as  the  development  of  drones  is  concerned.  In  the  early 
belief  in  Luci^ia  sine  conaibitii,  much  that  was  erroneous  was 
intermixed  with  a  prevision  of  the  truth;  nor  could  we  expect  at 
an  early  date  that  asexual  multiplication  {i.e.,  apart  from  ova  alto- 
gether) would  be  kept  distinct  from  what  we  now  mean  by 
parthenogenesis,  or  the  development  of  ova  without  union  with 
sperms.  In  1701,  Albrecht  observed  that  a  female  silkmoth, 
which  had  been  isolated  in  a  glass  case,  laid  fertile  eggs ;  and 
though  this  vv'as  for  long  discredited,  the  occasional  partheno- 
genesis of  this  insect  has  been  repeatedly  confirmed  by  com- 
petent observers. 

In  1745,  the  ingenious  Bonnet  drew  attention  to  what  is 
now  a  very  familiar  fact,  the  successive  generations  of  virgin 
plant-lice  or  Aphides.  Throughout  the  summer,  he  observed 
the  production  of  numerous  generations  of  these  little  insects, 
all  females,  necessarily  therefore  all  virgins,  and  yet  fertile.  So 
strange  did  the  fact  appear,  that  it  was  for  long  utterly  dis- 
credited. Reaumur  eluded  the  difficulty,  by  affirming  that  the 
Aphides  were  hermaphrodite  ;  but  Dufour  soon  proved  that  this 
was  erroneous,  though  he  could  only  confess  his  ignorance  in 
referring  the  phenomena  to  "  spontaneous  or  equivocal  gen- 
eration," in  which  "the  act  of  impregnation  was  in  no  degree 
concerned."  The  facts,  however,  were  repeatedly  re-observed. 
Kirby  and  Spence  admitted  them  as  incontestable,  but  could 
regard  them  only  as  "  one  of  the  mysteries  of  the  Creator,  that 
human  intellect  cannot  fully  penetrate." 


1  yo  THE    EVOLUTION    OF    SEX. 

Meanwhile  Schaffer  had  observed  the  occurrence  of  partheno- 
genesis in  minute  aquatic  crustaceans,  the  study  of  which  has 
since  shed  some  vivid  light  on  the  v/hole  subject.  Pastor 
Dzierzon  had  also  clipped  the  wings  of  queen -bees,  and  in 
thus  preventing  their  nuptial  flight  and  impregnation,  observed 
that  the  eggs  they  laid  developed  only  into  drones.  The  facts 
soon  began  to  be  recognised,  extended,  and  thought  over  by 
naturalists  of  the  standing  of  Owen  (1843),  Von  Siebold  (1856), 
and  Leuckart  (1858),  whose  conclusions  have  afforded  a  firm 
basis  for  the  abundant  subsequent  observation  and  speculation 
on  this  interesting  subject. 

§  2.  Degrees  of  Parthe?iogenesis. — If  we  start  then  with  Von 
Siebold's  definition  of  parthenogenesis,  as  the  power  possessed 
by  certain  female  animals  of  producing  offspring  without  sexual 
union  with  a  male,  it  will  clear  the  ground  to  notice,  in  the  first 
place,  the  numerous  different  degrees  in  which  this  development 
without  fertilisation  may  occur. 

{a)  Artificial  Parthenogenesis. — There  are  a  few  curious 
observations  which  go  to  show  that  in  exceptional  circumstances 
ova  may  develop  when  the  male  stimulus  is  replaced  by  some 
artificial  reagent.  These  observations  must  still  be  taken  ami 
grano  salis,  but  they  may  be  at  least  suggestive  of  further 
experiment.  Dewitz  observed  unfertilised  frog  ova  to  undergo 
segmentation  {sic)  in  corrosive  sublimate  solution.  In  some 
cases  one  division  occurred,  in  others  several ;  in  some  cases 
irregularly,  in  others  normally.  It  happened  both  when  the 
ova  were  left  in  the  reagent,  and  when  they  were  merely  dipped 
and  returned  to  water.  The  eggs  experimented  on  were  those 
of  the  two  common  frogs  Rana  fusca  and  R.  esculenta^  and  of 
the  tree-frog  {Hyla  arborea).  But  it  must  be  noted  that  Leuckart 
long  ago  noted  the  occurrence  of  spontaneous  division  in 
frog  ova.  Similarly,  Tichomiroff,  experimenting  with  the  un- 
fertilised ova  of  the  silkmoth,  which  are  occasionally  partheno- 
genetic,  was  surprised  to  observe  that  ova,  which  would  not  of 
themselves  develop  parthenogenetically,  might  be  induced  to 
do  so  by  certain  stimuli.  These  consisted  in  rubbing  the 
unfertilised  ova  with  a  brush,  or  in  dipping  them  for  two  minutes 
in  sulphuric  acid  and  then  washing  them.  In  both  cases,  he 
says,  a  percentage  of  the  ova  thus  artificially  stimulated  de- 
veloped. It  must  be  remembered  that  occasional  partheno- 
genesis occurs  in  this  insect,  and  all  that  Tichomiroff  did  was 
to    incite   this.     There  is    no   doubt  that    reagents    may   con- 


DEGENERATE  SEXUAL  REPRODUCTION.         171 

siderably  modify  ova ;  thus  the  brothers  Hertwig  showed  how  it 
was  in  this  way  possible  to  overcome  the  non-receptivity  of  the 
ovum  to  more  than  one  sperm.  Nor  can  one  forget  how  sexual 
reproduction  in  parasitic  fungi  tends  to  disappear,  being  ap- 
parently replaced  by  the  stimulus  afforded  from  the  waste  pro- 
ducts of  the  host.  In  a  similar  way,  the  multiplication  of 
cells,  so  frequently  associated  in  disease  with  the  presence  of 
bacteria,  has  been  referred  by  more  than  one  pathologist  to  the 
"  spermatic  influence "  of  these  micro-organisms,  or  of  the 
katastates  which  they  form. 

{^)  Pathological  Parthenogenesis. — It  has  very  occasionally 
been  noticed  in  higher  animals,  where  true  parthenogenesis  is 
wholly  unknown,  that  an  unfertilised  egg  starts  off  on  its  own 
resources  without  any  male  stimulus  whatever.  This  is  noted 
by  I>euckart  for  frog  ova,  by  Oellacher  for  hens'  eggs,  and  by 
Bischoff  and  Hensen  even  in  mammals.  Such  cases  must  be 
regarded  as  rare  abnormalities,  comparable  perhaps  to  patho- 
logical formations  which  not  unfrequently  take  place  in  the 
ovary,  and  it  is  hardly  necessary  to  say  that  in  no  case  did  the 
development  proceed  far.  Balfour  has  also  cited  a  remarkable 
observation  of  Greeff,  who  saw  unfertilised  ova  of  the  common 
starfish  developing  in  ordinary  sea  water,  in  a  perfectly  normal 
fashion,  only  more  slowly  than  usual. 

ic)  Occasional  Parthenogenesis.  —  In  some  of  the  lower 
animals,  which  are  not  themselves  normally  parthenogenetic, 
but  have  relatives  so  addicted,  occasional  parthenogenesis  has 
been  frequently  observed.  These  differ  from  the  above  cases, 
since  the  results  are  more  successful,  often  in  fact  reaching 
maturity,  and  also  in  this,  that  since  related  forms  are  partheno- 
genetic, the  "abnormality"  is  evidently  of  a  much  milder  type. 
The  common  silkmoth  is  a  good  example  of  this  occasional 
parthenogenesis,  which  certainly  occurs,  though  rare  both  in  the 
genus  and  family.  "  A  whole  series  in  insects,"  Weismann  says, 
"  reproduce  exceptionally  by  parthenogenesis,  for  instance  many 
butterflies,  but  that  never  to  the  extent  that  all  the  eggs  which 
an  unfertilised  female  lays  develop,  but  only  a  fraction,  and 
usually  a  very  small  fraction  of  the  total  number,  the  rest  perish- 
ing. Examples  of  successful  occasional  parthenogenesis  (to  the 
extent  at  least  of  producing  males)  are  furnished  by  those  worker 
bees,  wasps,  and  ants  which  exceptionally  become  fertile." 

{d)  Partial  Pai'thenogenesis. — The  queen-bee,  as  has  been 
already  mentioned,  is  impregnated  by  a  drone  in  her  nuptial 


172  THE    EVOLUTION    OF    SEX. 

flight.  The  sperms  thus  received  are  stored  up,  and  used  to 
fertihse  the  eggs  as  she  lays  them  in  the  cells.  Not  all  the  eggs, 
however,  but  only  those  which  will  produce  future  queens  or 
else  workers.  Other  eggs,  to  all  appearance  similiar,  are  un- 
fertilised, and  these,  as  Dzierzon  first  clearly  showed,  develop 
solely  into  drones.  We  cannot,  however,  say  that  the  absence  or 
presence  of  fertilisation  is  the  sole  difference,  though  if  fertilis- 
ation be  prevented  by  the  imperfect  development  of  the 
wings,  or  by  clipping  them,  the  queen  only  lays  drone  eggs. 
The  same  happens  when  she  is  old  and  her  store  of  male 
elements  exhausted,  or  when  the  sperm  receptacle  has  been 
removed.  Von  Sie])old  carefully  examined  the  eggs  from  drone- 
cells,  and  found  that  they  never  contained  spermatozoa.  Hensen 
notes  an  interesting  side  fact,  obviously  corroboratory,  that 
"  German  queen-bees,  fertilised  by  Italian  or  Cyprian  drones, 
produced  hybrid  females  but  pure  drones,  a  proof  that  on  the 
latter  the  sperm  does  not  operate."  Again,  it  sometimes  happens 
that  what  are  called  "  fertile  workers  "  crop  up,  which  in  con- 
sequence of  some  accident  or  misdirected  intention  in  the 
nutrition,  become  less  abortive  than  the  host  of  semi-females 
which  make  up  the  body  of  workers.  They  are  fertile  enough 
to  lay  eggs,  but  their  female  organs  do  not  seem  to  admit  of 
their  being  impregnated.  Certain  it  is  that  they  only  produce 
drones.  What  has  just  been  said  in  regard  to  bees,  is  also 
true  of  some  w^asps  and  ants. 

(e)  Seasonal  Parthenogenesis. — In  some  of  the  minute  aquatic 
crustaceans  (Cladocera),  popularly  included  under  the  general 
title  of  water-fleas,  parthenogenesis  only  occurs  for  a  season,  and 
is  periodically  interrupted  by  the  birth  of  males,  and  the 
occurrence  of  the  ordinary  sexual  reproduction.  Males  generally 
reappear  in  the  disadvantageous  conditions  of  autumn,  but 
Weismann  denies  that  there  is  a  direct  connection  between  these 
facts.  The  common  Aphides  are  parthenogenetic  for  a  succes- 
sion of  generations,  sometimes  as  many  as  fourteen,  throughout 
the  summer,  but  the  cold  and  hard  times  of  autumn  bring  back  the 
males  and  the  sexual  process.  The  fertilised  egg  lives  on  through 
the  winter,  and  develops  with  the  warmth  of  the  next  spring. 
By  keeping  up  the  temperature  and  nutritive  optimum  for  three 
or  four  years  in  the  artificial  summer  of  a  glass  case,  Reaumur 
and  Kyber  succeeded  in  rearing  as  many  as  fifty  continuous 
parthenogenetic  generations.  In  the  gall-wasps  {Cynipidie)  there 
is   usually   only   one   parthenogenetic  generation    between   the 


DEGENERATE  SEXUAL  REPRODUCTION.         173 

normal  sexual  reproductions,  but  in  many  insects  besides 
Aphides  there  are  several.  It  ought  to  be  noted,  that  the  parth- 
enogenetic  Aphides  are  hardly  at  the  same  structural  level  as  the 
females  which  are  fertilised  ;  but  as  the  differences  mainly  lie  in 
the  absence  of  certain  accessory  genital  organs,  there  is  no 
reason  for  regarding  the  parthenogenetic  forms,  as  some  have 
done,  as  larval. 

(/)  Juvenile  Parthenogenesis. — Cases  do  occur,  however, 
where  larval  forms  become  precociously  reproductive  (as  some- 
times happens  among  higher  organisms),  and  produce  offspring 
parthenogenetically.  Such  precocious  production  of  partheno- 
genetic ova  must  be  distinguished  from  the  entirely  asexual 
reproduction  exhibited  by  many  larvae.  No  very  firm  line  can 
indeed  be  drawn,  but  in  the  last  cases  no  cells  which  can  be 
called  ova  are  present.  In  1865  Professor  N.  Wagner  observed 
what  has  been  much  studied  since,  that  in  the  larvae  of  some  two- 
winged  or  dipterous  midges  {e.g.^  Miastoi-)^  the  cells  of  the  repro- 
ductive rudiment  develop  into  larvae  within  the  mother-larva's 
body.  The  mother  falls  victim  to  her  precocity,  for  the  brood 
of  seven  to  ten  larvae  literally  feed  upon  her  to  the  death.  They 
finally  leave  the  corpse  and  begin  life  for  themselves,  only  how- 
ever to  fall  themselves  victims  to  a  similar  fate.  The  process 
may  thus  go  on  for  several  generations,  during  which  the  ova, 
or  pseudova  as  some  would  insist  upon  calling  them,  become 
smaller  and  smaller.  Eventually  the  larvae  become  too  constitu- 
tionally poor  to  be  precociously  parthenogenetic,  and  develop 
into  adult  midges — male  and  female,  the  latter  producing  how- 
ever only  a  few  eggs. 

In  another  dipterous  insect  known  as  Chironomus.,  the  ova 
begin  to  be  produced  at  a  very  early  stage,  are  laid  just  at  the 
time  when  the  larval  life  ends,  and  develop  parthenogenetically. 
According  to  Jaworowski,  by  the  rupture  of  the  ovarian  mem- 
brane the  ova  fall  into  the  body-cavity,  where  the  abundant 
nutritive  stimulus  takes  the  place  of  fertilisation.  Juvenile 
parthenogenesis  is  also  said  by  Von  Siebold  to  occur  among 
the  Strepsiptera,  little  insects  which  infest  bees. 

( o")  Total  Faiilienogenesis. — Lastly,  in  some  of  the  minute 
aquatic  crustaceans  and  in  many  rotifers  no  males  have  ever 
been  found.  There  is  every  probability  that  the  parthenogenesis 
is  thus  total ;  and  as  the  numbers  are  abundant,  it  has  apparently 
been  established  without  detriment  to,  at  least,  the  continuance 
of  the  species. 


174  THE    EVOLUTION    OF    SEX. 

§  3.  Occurrence  of  Pa7'thenogenesis. — In  these  distinct  sets  of 
animals — rotifers,  crustaceans,  and  insects — parthenogenesis  has 
become  a  confirmed  physiological  habit. 

{a)  Take  first  the  curious  little  rotifers,  or  wheel-animalcules,  which 
abound  both  in  fresh  and  salt  water.  They  are  usually  placed  in  the 
chaotic  alliance  of  worm-types,  and  have  long  been  famous  for  their  alleged 
power  of  surviving  prolonged  desiccation.  With  one  or  two  exceptions 
the  males  are  markedly  different  from  the  females,  and  are  usually  small  and 
degenerate.  In  one  group  {P/iilodijiadiv)  the  females  have  two  ovaries, 
while  males  have  never  been  found.  They  have  dwindled  out  of  existence. 
In  the  rest  the  females  have  one  ovary,  part  of  which  has  degenerated  into 
a  yolk-gland,  and  small  males  occur.  These  are  quite  superfluous  as  mates, 
however,  for  parthenogenesis  prevails.  Even  when  impregnation,  which  is 
a  peculiarly  random  process,  occurs,  the  sperms  appear  to  miss  their  mark, 
and  to  perish  in  the  body-cavity.  The  numbers  keep  up,  notwithstanding, 
so  that  we  have  here  an  entire  class  where  parthenogenesis  has  firmly 
established  itself. 

[b)  Among  crustaceans,  parthenogenesis  is  restricted  to  the  lower  orders, 
viz.,  branchiopods  and  ostracods.  In  the  former,  it  is  exhibited  by  the 
brine-shrimp  Arteniia  and  the  common  fresh-water  Aptis  in  one  division  ; 
by  daphnids  {e.g.,  DapJuiia  and  iMonia,  common  "water-fleas")  in  the 
other.  In  ostracods,  some  species  of  the  common  Cypris  are  partheno- 
genetic.  If  a  female  water-flea,  say  Daphnia,  be  isolated  from  birth, 
she  becomes  the  mother  of  an  abundant  progeny  of  females.  Males  and 
sexual  reproduction  do  however  eventually  return,  and  the  same  is 
probably  true  of  the  majority.  Among  three  thousand  specimens  of  the 
brine-shrimp  only  one  male  occurred  ;  while  Von  Siebold  repeatedly  in- 
vestigated every  member  of  a  colony  of  Apus,  once  over  five  thousand  in 
number,  without  finding  a  single  male.  At  other  times  he  found  one  per 
cent.,  while  in  certain  unknown  conditions  (probably  when  food  is 
scarce  and  life  generally  unfavourable)  the  males  may  be  developed  in 
crowds. 

In  the  daphnids,  which  have  been  so  successfully  studied  by  Weismann, 
the  facts  are  more  complex.  There  are  two  kinds  of  eggs — winter  and 
summer  ova.  The  former  are  large,  thick  shelled,  capable  of  resisting 
drought  and  the  like,  and  of  remaining  long  latent.  They  only  develop  if 
fertilised,  and  always  produce  females.  In  every  way  they  are  highly 
anabolic  ova.  The  summer  eggs,  on  the  other  hand,  are  smaller,  and  thinner 
in  the  shell.  They  can  develop  without  fertilisation,  and  that  is  indeed 
in  some  cases  physically  impossible.  Males  are  produced  from  summer  eggs 
alone.  They  usually  ajipear  in  autumn,  when  life  is  becoming  harder,  or 
the  conditions  more  katabolic. 

In  the  little  cyprids  the  reproductive  relations  are  very  varied.  Thus  in 
Cypris  ovum  and  Notodronius  7/ionac/ius  the  males  are  abundant  all  the  year 
round,  and  parthenogenesis  is  unknown.  In  other  species,  e.g.,  Candona 
Candida,  the  males  are  still  frequent,  but  parthenogenesis  nevertheless 
occurs.  Lastly,  parthenogenesis  prevails  in  some  cases,  like  Cypris  fusca 
and  C.  piibera,  and  the  males  are  rare,  appearing  usually  in  spring. 

(f)  In  insects,  as  we  have  seen,  the  degrees  of  parthenogenesis  are  very 
varied  ;  so  too  is  the  systematic  position  of  the  forms  in  which  normal 
parthenogenesis   occurs.      Two   butterflies    {Psyche  helix   and   Solenobia, 


DEGENERATE  SEXUAL  REPRODUCTION. 


175 


2  sp.)  and  a  beetle  {Gastrophysa) ',  some  coccus-insects  and  Aphides; 
certain  saw-flies  [Tenihredinidcc)  and  gall-wasps  (Cj/^z^/V/^?),  are  normally 
parthenogenetic.  In  the  butterflies  just  noticed,  the  males  seem  to  dis- 
appear for  a  stretch  of  years,  and  the  species  gets  on  without  them.  The 
male  of  Psyche  helix  is  very  rare,  and  was  for  long  unknown.  When  the 
males  are  developed  in  Solenohia  ti-inqnetrella,  it  is  interesting  to  notice 
that  they  may  predominate  in  numbers  over 
the  females.  A  whole  brood  may  be  male ;  they 
are  brought  back  with  a  rush.  About  a  score 
of  moths,  including  the  silkmoth  {Bomhyx 
mori)  and  death's-head  {Sphinx  atropos)  have 
been  known  to  exhibit  casual  parthenogenesis  ; 
but  the  beetle  above  noticed  stands  alone. 
Bassett,  Adler,  and  others,  have  demonstrated 
an  interesting  alternation  of  parthenogenesis 
and  ordinary  sexual  reproduction  in  numerous 
gall-wasps.  Forms  which  had  been  regarded 
as  quite  distinct,  and  had  received  different 
generic  titles,  have  been  shown  in  about  a  score 
of  cases  to  be  merely  the  parthenogenetic  and 
normal  forms  of  the  same  insects.  From  a 
winter  gall  the  parthenogenetic  form  emerges 
which  produces  a  summer  gall.  In  this  a  sexual 
form  is  produced,  which  eventually  gives  rise 
to  the  winter  gall. 

§  4.  Parthenogenesis  in  Plants. — The  pas- 
sive bias  is  so  strong  in  plants,  that  it  is  easy 
to  understand  the  rarity  of  parthenogenesis. 
The  egg-cell  which  develops  of  itself  must  re- 
tain the  stimulus  which  the  male  element  in 
other  cases  supplies.  It  is  natural,  then,  that 
what  predominates  in  the  active  rotifers  should 
be  uncommon  in  the  sleeping  plants.  In  some 
of  the  flowering  plants,  what  looked  like  par- 
thenogenesis has  repeatedly  been  described, 
especially  in  regard  to  a  native  of  New  PIol- 
land,  known  as  Cixlebogym.  When  cultivated 
in  Europe,  the  male  flowers  degenerate,  and 
according  to  Braun  and  Hanstein  disappear. 
Yet  fertile  seeds  are  produced.  Karsten  found, 
however,  that  stamens  often  persisted  ;  while 
Strasburger  has  shown  that  what  developed 
were  not  true  egg-cells,  but  adventitious  growths 
from  cells  outside  the  embryo-sac.  The  same 
is  true  of  some  other  cases.  Dr  A.  Ernst  has 
recently  described  what  he  calls  true  partheno- 
genesis in  a  Menisperm  found  by  him  in  Caracas, 
and  named  Disciphania  Ernstii.  "Female  plants,  which  bore  no  male 
flowers,  and  which  were  grown  perfectly  isolated  where  there  was  no  pos- 
sibility of  the  access  of  pollen  from  another  plant,  produced  in  three  succes- 
sive years  an  increasing  number  of  fertile  fruits." 

In    the   lower   plants,    however,    there   is   no   doubt   on    the    subject. 


Owen's  figure  of  the  Genera- 
tions of  Aphides.  At  the 
base  an  individual  arises 
from  a  fertilised  egg-cell  ; 
this  gives  origin  partheno- 
genetically  to  a  brood,  and 
so  on  through  a  succession 
of  generations.  At  the  top 
the  male  and  female  forms 
reappear,  and  sexual  re- 
production returns.  At  the 
side  an  earlier  appear- 
ance of  sexual  forms  is 
suggested. 


176  THE    EVOLUTION    OF    SEX. 

Parthenogenesis  frequently  occurs  as  one  of  the  stages  in  the  degeneration 
of  sexual  reproduction.  It  has  been  casually  observed  of  a  species  of 
the  stonewort  {Chard),  that  when  grown  in  certain  waters  the  male 
organs  disappear,  yet  the  plants  continue  multiplying.  More  interesting 
are  the  Fungi.  To  illustrate  sexual  degeneration,  De  Bary  gives  a  series 
from  Fungi  like  those  which  kill  the  salmon  and  potato  {Saprolcgniic  and 
Peronosporecr).  What  happens  first,  is  the  degeneration  of  the  male  organs. 
The  katabolic  sex  from  beginning  to  end  is  the  more  unstable.  The  male 
function  goes  first,  but  the  form  remains  after  the  reality  has  ceased. 
After  a  while,  that  is  in  related  species,  the  form  goes  too.  Sometimes 
the  function  is  changed,  and  the  male  organs  become  sort  of  protective 
sheaths.      His  series  may  be  briefly  summed  up. 

(i.)  In  Pythiiim,  the  male  organ  discharges  most  of  its  protoplasm  into 
the  female, — the  usual  story. 

(2.)  In  Phytoplitliora,  only  a  very  small  portion  is  thus  given,  and  we 
may  almost  say  asked,  for  there  are  curious  demand  and  supply 
arrangements  and  compulsions  between  the  male  and  female  organs 
in  these  Fungi. 

(3.)  In  Peronospora,  there  is  no  perceptible  passage  of  protoplasm  from 
male  to  female,  though,  without  going  back  to  the  "aura 
seminalis,"  we  may  allow  the  possibility  of  subtle  osmosis. 

(4.)  In  some  Saprolegiiicv,  there  are  indeed  the  usual  antheridia  or  male 
organs,  which  are  directed  towards  the  female  organs,  but  do  not 
open.     The  "  explosive"  character  is  diminishing. 

(5.)  In  others,  the  male  organs  never  get  near  the  female. 

(6.)  In  others,  there  are  no  male  organs  at  all,  but  the  female  cells 
develop  as  usual. 

Parthenogenesis  is  thus  reached,  as  the  result  plainly  of  a  degenerative 
process.  We  can  follow  the  story  further,  however,  forestalling  for  the 
moment  the  subject  of  the  next  chapter.  The  male  organ  has  degenerated, 
we  have  seen,  while  the  female  organ  holds  on  its  course.  Put  this  is  not 
always  so ;  in  many  cases  it  follows  suit,  and  asexual  reproduction 
remains. 

Now  why  should  these  Fungi  among  plants  exhibit  numerous  instances 
of  parthenogenesis  ?  The  more  intimate  the  parasitism,  the  more  degener- 
ate the  sexual  reproduction,  and  all  trace  of  it  is  often  lost.  The  Fungus 
fertilises  itself  from  its  host.  In  the  Fungus  on  the  coffee  plant,  for 
example,  the  stimulus  of  fertilisation  is  replaced  as  it  were  by  an  "essence 
of  coffee." 

Male  parthenogenesis,  paradoxical  as  it  sounds,  is  really  exhibited 
among  lowly  alga:.  Thnt  is  to  say,  a  small  spore  (or  male-cell)  which 
normally  unites  with  a  larger  and  more  quiescent  one  (or  female-cell),  may 
occasionally  start  developing  on  its  own  resources.  The  result,  however, 
is  poor  enough.  As  those  spores  are  on  the  border  line  between 
asexuality  and  differentiated  sex-elements,  the  retention  of  a  vegetative 
power  of  division  even  by  the  incipient  male-cell  is  not  surprising.  Nor 
must  it  be  forgotten  that  the  mother-spcrm-cell  itself  has  a  power  of 
parthenogenetic  development.  It  divides,  like  its  homologue  the  ovum, 
into  a  ball  of  cells,  but  having  none  of  the  conservative  coherence  of  the 
latter  breaks  up  into  spermatozoa.  It  is  exactly  comparable  to  the 
interesting  Protozoon  [Magospluera)  which  Hoeckcl  saw,  which  did  its  best 
to  get  beyond  the  Protozoa,  but  failed  as  soon  as  it  had  succeeded.     A 


DEGENERATE  SEXUAL  REPRODUCTION. 


^77 


single  infusorian-like  cell  divided  into  a  ball  of  cells,  but  the  ball  had  no 
coherence  and  broke  up  into  infusorians  once  more. 

§  5.  The  Offspring  of  Parthenogenesis. — The  fate  of  parthenogenetic 
ova  is  very  diverse.  They  may  all  perish,  or  all  succeed  ;  they  may  turn 
out  wholly  males  or  wholly  females.  Hensen  notes  the  following  suggestive 
series,  with  decreasing  reproductive,  as  opposed  to  constitutional,  energy 
at  each  level : — 

(l.)  Hermaphrodites,  then  only  females. 

{2.)  Series  of  females,  then  mixed  brood. 

(3.}  Several  females,  mixed  brood,  then  only  males. 

(4.)  Series  of  mixed  broods,  then  males,  or  death  of  ova. 

{5.)  Mixed  brood,  with  much  mortality. 

(6.)  Males  only. 

(7.)  Development  only  for  a  few  stages. 
Rolph  has  a  different  arrangement,  but  the  same  idea  : — 

(1.)  Exceptional  parthenogenesis  with  uncertain  result  (^.^.,  Silkmoth). 

(2.)  Normal,  producing  males  only  (female  solely  from  fertilised  ova) 
{e.g..  Bees). 

(3.)  Mostly  males,  with  occasional  females  {e.g.,  Nematus). 

(4.)  Mostly  females,  with  exceptional  or  periodic  males  {e.g.,  Apus, 
Artemia). 

(5.)  Only  female,  males  unknown  {e.g.,  many  Rotifers). 

That  parthenogenetic  ova  should  develop  with  such  diverse  results  is 
not  at  all  surprising.  The  absence  of  fertilisation  removes  one  of  the 
factors  determining  sex  ;  but  food,  temperature,  age  of  ovum,  &c.,  remain, 
and  produce  bias  now  to  one  side,  now  to  the  other.  To  this  we  shall 
presently  return  ;  meanwhile  the  facts  of  offspring  may  be  more  clearly 
expressed  thus  : — 

Result.  Example. 


O 
u 

'^ 
ux 
o 
o 

IS 

K 
H 
Pi 
< 


'Nil        .        .        .        . 
Partial  and  pathological  development 
Great  mortality  in  a  mixed  brood.    . 
i  's  alone        ...... 

i  's  mostly,  a  few  ?  's    . 

cJ  's  and   9  's  (one  generation) 
(J 's,  and  more  than  a  few   ?  's 
9  9  9   (a  succession),  then  a  predomin- 
ance of  (5  's 
9  9  9,  then  equal  numbers  of  6  's  and  9  's 
9  9  9,  then  a  minority  of  i  's  among  9  's 


9  9  ,  very  rare   (5  's 

9  9  ,  non-functional   6  's  among 

9  9  >  ad  infinitum,  no   cJ  's 


9's 


9  9 
9  9 
19  9 

§  6.  Effects  of  Parthenogenesis. — Since 
dominant    in   rotifers,  and  well  established 
and  plant-lice,  it  is  very  plain  that  whatever 
anything  but  prejudicial  to  numbers.     An 

M 


Most  organisms. 
Rarities  mentioned. 
Many  insects. 
Hive-bee    and    some 

other  forms. 
Nematus     (allied     to 

bee). 
Most  gall- wasps. 
Some  saw-flies. 

Some  water-fleas. 
Solenobia  sometimes. 
Aphides ;  some  water- 
fleas. 
Many  water-fleas. 
Most  rotifers. 
Many  rotifers. 

parthenogenesis  is 

among  water-fleas 

else  it  affects,  il  is 

aphis  will  continue 


178  THE    EVOLUTION    OF    SEX. 

for  days  producing  a  viviparous  brood,  at  the  rate  of  one  per 
hour;  the  offspring  soon  begin  themselves  to  multiply ;  and 
Huxley  calculates,  that  if  this  continued  for  a  year  without 
mortality,  a  single  aphis  would  be  the  ancester  of  a  progeny 
which  would  weigh  down  five  hundred  millions  of  stout  men  ! 
Not  gardeners  only  have  cause  for  gratitude  that  climate  and 
enemies  prevent  such  untoward  increase.  But  there  are  other 
desiderata  besides  numbers.  Can  it  be  said  that  parthenogenesis 
favours  the  general  life  and  progress  of  the  species?  It  will 
be  at  once  recognised  that  rotifers,  brine-shrimps,  water-fleas, 
aphides,  coccus-insects,  and  so  on,  are  relatively  low  forms. 
Only  two  or  three  butterflies  and  one  beetle  are  parthenogenetic. 
Higher  up  in  the  scale  virgin  birth  never  occurs  except  in  a 
very  partial  and  pathological  degree.  But  we  can  go  further. 
More  than  one  of  the  old  naturalists,  and  in  recent  years 
Brooks,  Galton,  Weismann,  and  others,  have  laid  emphasis  on 
the  value  of  fertilisation  as  a  fountain  of  change.  To  Weismann 
the  intermingling  of  the  male  and  female  "  germ-plasmas  "  in 
fertilisation  is  really  the  sole  source  of  variation.  That  it  is  a 
source,  all  will  admit.  If  it  be  removed  therefore,  as  in  rotifers, 
the  species  will  be  so  much  the  less  likely  to  progress.  Weis- 
mann holds  that  it  will  not  progress  at  all ;  and  though  we 
should  not  go  quite  so  far,  we  are  bound  to  allow  that  the 
establishment  of  parthenogenesis  is  a  handicapping  of  evolution. 
We  cannot,  however,  follow  Weismann  in  his  next  step.  If 
all  change  springs  from  the  sexual  intermingling,  the  rotifer 
species  cannot  change  at  all.  They  cannot  go  forwards,  nor 
yet  backwards.  Having  attained  to  a  physiological  state  when 
males  became  superfluous,  they  remain  171  statu  quo.  So  he 
emphasises  that  superfluous  organs,  such  as  the  sperm-receptacle, 
do  not  become  rudimentary  in  parthenogenetic  species, — "  rudi- 
mentary organs  can  only  occur  in  species  with  sexual  reproduc- 
tion." This  is  a  corollary  of  Weismann's  contention  that  no 
individually  acquired  characters,  either  plus  or  minus,  can  be 
transmitted,  and  that  the  sexual  intermingling  is  the  sole  source 
of  change  affecting  the  species.  Were  the  main  propositions 
proven,  the  corollary  would  follow,  but  there  are  still  many 
dissentient  voices.  Without  going  into  the  general  question 
at  present,  let  us  take  the  corollary  by  itself,  (i.)  Cases  where 
males  are  quite  unknown  are  comparatively  few ;  in  most 
cases  they  reai)pear  at  intervals.  It  is  not  possible,  therefore, 
as  Weismann  will  allow,  to  be  certain  that  the  sperm-receptacle 


DEGENERATE  SEXUAL  REPRODUCTION.         I 79 

becomes  superfluous  to  the  species.  (2.)  He  also  allows  that  it 
does  degenerate  in  the  summer  aphides,  where  the  periodic 
disappearance  of  males  is  well  known.  (3.)  In  spite  of  the 
absence  or  else  futility  of  impregnation  in  rotifers,  we  find  the 
males  obviously  in  process  of  degeneration. 

In  conclusion,  we  believe  with  Weismann  and  others,  that 
the  absence  of  fertilisation  is  a  minus  in  evolution,  but  see  no 
warrant  for  supposing  that  it  absolutely  precludes  either  pro- 
gress or  the  reverse.  The  power  of  parthenogenetic  birth  has 
two  different  results.  (i.)  The  female  cell  has  a  certain 
maleness  about  it ;  it  retains  the  stimulus  which  the  male  ele- 
ment usually  affords ;  the  species  will  therefore  be  frequently 
of  active  male-like  habit,  e.g.,  rotifers  and  water-fleas.  (2.)  On 
the  other  hand,  the  long  continued  production  of  females 
means  an  anabolic  preponderance,  a  weighting  of  the  species ; 
and  this  is  seen  in  the  sluggish  plant-lice,  coccus  insects,  and 
the  Hke. 

§  7.  Peculiarity  of  the  Farthe?iogenetic  Ova. — Before  a  theory 
of  parthenogenesis  is  sought,  the  natural  question  arises,  Are 
these  eggs  that  develop  of  themselves  in  any  way  peculiar? 
{a)  For  a  while  it  was  supposed  {e.g.,  by  Balfour)  that 
parthenogenetic  ova  did  not  form  polar  globules,  and  the 
theory  based  upon  that  regarded  the  retention  of  these  bodies 
as  taking  the  place  of  fertilisation.  The  demonstrated  occur- 
rence of  one  polar  globule  in  several  parthenogenetic  eggs 
partially  demolished  this  theory,  and  it  is  only  within  the  last 
two  or  three  years  that  it  has  been  restated  in  accurate  form. 
{b)  Simon  shrewdly  points  out,  that  in  some  of  the  most 
marked  cases  of  parthenogenesis  the  sex-cells  are  insulated 
from  the  body  at  a  very  early  stage.  This  is  notably  so  in 
those  midges  which  reproduce  parthenogenetically  even  before 
maturity.  It  is  certainly  striking  that  these  forms  should 
unite  an  extreme  earliness  in  the  embryonic  separation  of 
the  germ-cells  with  a  most  precocious  reproduction.  These 
germ-cells  are  ova  which  have  a  much  less  circuitous  history 
than  in  most  cases ;  they  have  far  fewer  cell-divisions  behind 
them,  they  have  thus  a  reserve  power  of  division  which  other 
ova  have  not ;  they  are  able,  in  fact,  to  develop  of  themselves. 
This,  unfortunately,  is  not  known  to  be  true  of  some  of  the  most 
signal  cases  of  parthenogenesis  {e.g.,  rotifers) ;  but  it  is  true  of 
some,  and  that  to  a  greater  extent  than  was  known  when  Simon 
wrote.     On  the  other  hand,  some  forms  where  parthenogenesis 


l8o  THE    EVOLUTION    OF    SEX. 

is  unknown  {e.g.,  leeches  and  Sagitta),  also  exhibit  the  same 
early  differentiation  of  germ-cells,  so  that  we  can  only  look 
upon  the  fact  as  one  of  the  auxiliaries  of  parthenogenesis. 

(c)  The  peculiarity  of  parthenogenetic  ova,  which  has  of  late 
attracted  much  attention,  is  that  they  extrude  only  one  polar 
cell, — not  two,  like  other  eggs.  This  discovery  is  due  to  Weis- 
mann,  who,  with  the  assistance  of  Herr  Ischikawa,  has  verified  it 
in  about  a  dozen  species,  Leptodora  hyalifia,  Sida  crystallina, 
Cypris  reptans.,  and  other  water-fleas.  Blochmann  has  also 
corroborated  Weismann's  discovery,  in  his  observations  on 
aphides.  What  theoretical  importance  Weismann  attaches  to 
the  fact  will  be  immediately  noticed."^ 

§  8.  Theory  ofFarthenogefiesis. — We  may  begin  with  Balfour's 
view  of  the  case,  though  that  of  Minot  has  the  priority.  "  The 
function  of  forming  polar  cells  has  been  acquired  by  the  ovum 
for  the  express  purpose  of  preventing  parthenogenesis."  If  they 
were  not  formed,  parthenogenesis  would  normally  occur.  This 
is  expressed  in  curiously  teleological  language,  but  the  main 
idea  is  clear  enough, — the  retained  polar  cells  replace  the  sperm 
nucleus.  It  is  only  necessary  to  change  cells  into  cell  to  make 
it  reasonable  to-day.  One  must  not  forget,  however,  that  in 
higher  animals,  where  parthenogenesis  is  unknown,  polar  cells 
have  not  been  found  often  as  yet,  nor  ever  seen  in  birds  and 
reptiles.  And  one  would  fain  get  further  back  still,  and  know 
7vhy  only  one  polar  globule  is  formed  in  parthenogenetic  ova. 

"  In  accordance  with  Minot's  hypothesis  of  sexuality,  it 
might  be  assumed  that  in  parthenogenetic  ova  the  male  element 
was  retained,  and  that  the  cell  remained  a  true  asexual  cell,  and 
did  not  become  a  sexual  element."  "  Blochmann  and  Weis- 
mann have  shown  that  this  is  the  case,  by  their  discovery  that 
in  parthenogenetic  ova  only  one  polar  globule  is  formed,  while 
there  are  always  two  in  ova  which  are  impregnated  ;  hence  it  is 
probable  that  one  polar  globule  (by  hypothesis,  male)  is  re- 
tained." 

Minot's  words  are  not  beyond  criticism  either,  though  they 
are  not  teleological.  An  ovum  which  retains  a  male  element 
is  not  happily  described  as  remaining  asexual ;  it  would  be  better 
to  call  it  a  case  of  intra-cellular  hermaphroditism.  Nor  can  it 
yet  be  said  that  there  are   always    two  polar  globules  in  ova 

*  Blochmann,  however,  claims  to  have  demonstrated  the  formation  of 
tzvo  polar  bodies  in  those  unfertilised  eggs  which  are  to  give  birth  to  drones. 


DEGENERATE  SEXUAL  REPRODUCTION.         l8l 

which  are  impregnated.  The  discovery  referred  to  is  histori- 
cally Weismann's,  while  a  corroboration  is  due  to  Blochmann. 
It  is  more  important,  however,  to  notice  how  Minot  cleverly 
adapts  himself,  and  rightly  too,  to  increased  knowledge  of  the 
facts.  The  parthenogenetic  ovum  only  retains  one  polar  globule, 
— one  male  element  is  enough;  two  would  be  "polyspermy," 
which  is  abhorred. 

There  was  no  fear  that  Rolph  would  indulge  in  teleology, 
rigid  necessitarian  as  he  was.  Parthenogenesis  of  ova  was  to 
him  the  more  natural  process,  the  sperm  a  subsequent  impor- 
tation. "  There  is  for  the  ovum  a  certain  minimal  mass,  which 
must  be  surpassed  if  it  is  to  develop  at  all ;  and  a  second  minimum, 
which  the  ovum  must  attain,  if  a  female  is  to  be  produced." 
Abundant  nutrition  of  the  ovum  tends  to  parthenogenesis,  pro- 
ducing male  offspring,  as  the  lower  stage ;  but  if  the  second 
limit  be  attained,  resulting  in  females.  In  the  opposite  direc- 
tion, if  the  ovum  have  fewer  resources,  it  requires  to  be  fertilised. 
Females  or  males  will  again  result  according  to  the  state  of  the 
elements.  If  no  fertilisation  occur,  the  dependent  ovum  must 
of  course  die.  Rolph  is  always  suggestive,  but  he  erred  in 
regarding  the  sex-elements  too  quantitatively,  in  missing  the 
qualitative  antithesis  of  sex,  and  the  opposition  observed  in 
cell-division. 

{d)  Strasburger  also  lays  emphasis,  in  a  subtler  and  more 
technical  way,  on  nutritive  conditions.  "  In  the  rare  cases  of 
parthenogenesis,  specially  favourable  nutritive  conditions  may 
counteract  the  lack  of  nuclear  plasma."  He  notes  three  dif- 
ferent ways  in  which  this  may  happen,  and  also  inclines  to 
believe  that  retention  of  polar  globules  would  favour  partheno- 
genetic development.  It  is  important  to  notice  how  two 
naturalists,  so  very  different  in  their  manner  of  attacking  a 
subject  as  Rolph  and  Strasburger  are,  come  to  this  conclusion 
at  least  in  common,  that  favourable  nutritive  conditions  favour 
parthenogenesis.  AH  the  cells  in  the  body  tend  to  multiply, 
the  ova  retaining  this  power  develop  embryos. 

{e)  Weismann  has  a  peculiar  right  to  be  heard  on  the  nature  of  partheno- 
genesis. For  not  only  has  he  been  for  many  years  an  investigator  of  the  tiny 
daphnids  or  water-fleas,  but  he  has  recently  made  the  important  discovery, 
already  noticed,  that  parthenogenetic  ova  extrude  only  one  polar  globule. 
There  has  not  been  time  yet  to  prove  that  this  is  an  absolute  fact,  but  the 
probabilities  are  strong  that  it  is.  Before  stating  his  theory,  it  is  necessary 
to  remember  that  the  "germ-plasma"  of  Weismann  is  a  specific  and  essen- 
tial portion  of  the  nucleus  of  ovum  or  sperm,  part  of  which  keeps  up  the 


162  THE    EVOLUTION    OF    SEX. 

continuity  of  heredity,  by  passing  intact  into  the  reproductive  cells  of  the 
next  generation.  Besides  this  all-important  "  germ-plasma,"  the  nucleus 
of  the  ovum  contains,  according  to  Weismann,  an  "  ovogenetic  nuclear 
plasma,"  which  is  of  no  direct  importance  in  development,  but  is  useful  to 
the  ovum  simply  as  an  ovum.  It  is  the  substance  which  is  supposed  to 
have  to  do  with  the  general  upbuilding  of  the  egg-cell,  with  the  accumu- 
lation of  yolk,  secreting  of  membranes,  and  the  like. 

"  The  first  polar  body  implies  the  removal  of  the  ovogenetic  nuclear 
plasma,  which  has  become  superfluous  when  the  egg  has  attained  maturity. 
The  second  polar  body,  on  the  other  hand,  implies  the  removal  of  a  portion 
of  the  germ-plasma  itself.  This  is  so  effected  that  the  number  of  ancestral 
elements  {Ahnen-idioplasmen)  which  compose  it  is  reduced  to  a  half.  A 
similar  reduction  must  also  take  place  in  the  number  of  the  male  germ- 
elements. 

"  Parthenogenesis  occurs  when  the  entire  sum  of  the  ancestral  elements 
persists  in  the  nucleus  of  the  ovum.  Development  by  fertilisation  demands, 
however,  that  half  of  these  ancestral  elements  m.ust  first  be  extruded  from 
the  ovum,  whereupon  the  remaining  half,  in  uniting  with  the  sperm  nucleus, 
regains  the  original  number. 

"  In  both  cases  the  beginning  of  development  depends  upon  the  presence 
of  a  definite,  and  indeed  similar  mass  of  germ-plasma.  In  the  ovum  which 
requires  ferlilisation,  this  is  afforded  by  the  importation  of  the  sperm-nucleus, 
and  development  follows  on  the  heels  of  fertilisation.  The  parthenogenetic 
ovum  already  contains  the  necessary  mass  of  germ-plasma,  and  this  becomes 
active  as  soon  as  the  single  polar  body  has  freed  the  ovum  from  the  ovo- 
genetic nuclear-plasma." 

Now  if  it  be  true  that  a  constant  difference  between  an  egg  which  can 
develop  of  itself  and  one  that  cannot,  is  that  the  former  extrudes  one  tiny 
cell,  and  the  latter,  so  far  as  yet  observed,  two,  Weismann  must  be  right  in 
emphasising  that  part  at  least  of  the  secret  of  parthenogenesis  lies  here. 
Partly  hidden  still,  however,  if  one  dare  ask  what  there  is  about  the  par- 
thenogenetic ovum  which  limits  its  primitive  budding  to  once  instead  of 
twice.  Not  altogether  so  subversive  of  Minot's  theory  either,  as  Weismann 
would  make  out.  Minot,  as  we  saw,  accepts  ihe  facts,  but  ingeniously 
supposes  that  the  polar  element  retained  in  parthenogenetic  ova  is  a  male 
element.  It  is  necessar)',  however,  to  examine  Weismann's  theory  more 
closely,  not  only  in  its  direct  relation  to  the  problem  of  parthenogenesis, 
but  because  of  its  postulates,  which  run  so  directly  counter  to  our  reading 
of  the  phenomena  of  sex. 

(i.)  Weismann's  theory  obviously  differs  very  emphatically  from  those 
previously  suggested.  The  first  polar  body  is  no  skimming  of  antagonistic 
male  material ;  the  very  reverse,  it  is  an  extrusion  of  ovogenetic  nuclear 
material  which  had  to  do  with  the  upbuilding  of  the  ovum,  an  emphatically 
female  function.  Nor  is  the  second  polar  extrusion  in  any  way  an  expulsion 
of  male  elements  ;  it  is  a  giving  away  of  some  of  the  precious  germ-plasma, 
the  bearer  of  hereditary  characteristics.  Furthermore,  even  the  sperm 
nucleus  is  in  no  peculiar  sense  male  material  ;  it  might  as  well  be  another 
ovum-nucleus.  It  has  only  a  quantitative  value,  to  restore  to  the  nucleus 
of  the  ovum  an  amount  of  germ-plasma  equivalent  to  that  which  has  been 
so  recklessly  squandered. 

(2.)  But  Weismann's  theory,  based  on  the  observation  of  facts,  is  in 
itself  full  of  hypotheses.     This  distinction  between  ovogenetic  and  germ- 


DEGENERATE  SEXUAL  REPRODUCTION.         1 83 

plasma  within  the  germinal  vesicle  is  an  unverirtable  myth.  That  the  first 
polar  body  is  an  extrusion  of  one  kind  of  nuclear  substance,  and  the  second 
something  quite  different,  is  another  unproved  hypothesis.  Were  the  extru- 
sions markedly  different,  one  might  believe  it,  but  they  are  the  same. 
When  a  large  cell  divides  very  unequally,  as  in  polar  body  formation,  there 
is  some  warrant  for  supposing  that  the  little  bud  is  different  from  the  large 
cell  ;  but  that  two  successive  divisions,  entirely  similar  in  character,  are 
conspicuously  different,  requires  faith.  It  is  allowed  by  all  that  each  polar 
division  lessens  the  mass  (not  the  number)  of  the  chromatin  elements  in 
the  nucleus  by  a  half,  but  so  far  as  nucleus  is  concerned  there  is  nothing 
whatever  to  show  that  the  first  division  is  qualitatively  different  from  the 
second.  The  first  may  have  more  cell-substance  extruded  along  with  it, 
and  the  second  may  be  rather  a  nuclear  than  a  cell-division,  but  as  regards 
"plasma"  the  two  are,  so  far  as  the  facts  go,  absolutely  alike.  The 
second  division  also  follows  on  the  heels  of  the  first  without  the  inter- 
vention of  the  usual  resting  stage.  Nor  of  course  is  there  any  proof  that 
a  parthenogenetic  ovum  does  not  part  with  half  its  "  germ-plasma"  in  the 
first  division.  The  distinction  between  the  two  kinds  of  nuclear  plasma 
is,  in  plain  words,  a  myth. 

(3.)  Weismann's  pre-occupation  with  questions  of  inheritance  has  given 
a  bias  to  his  theory,  making  it  morphological  rather  than  physiological. 
A  given  quantum  of  germ-plasma,  he  says,  fits  the  ovum  to  develop.  The 
parthenogenetic  ovum  has  this  and  keeps  it.  The  ordinary  ovum  has  it 
too,  but  extrudes  it,  to  get  it  back  again  from  another  source.  If  this  is 
all  the  sperm  does,  one  cannot  help  wondering  that  such  a  circuitous  pro- 
cess could  ever  arise.  The  entrance  of  the  sperm  must  be  looked  at  in 
two  ways, — (a)  It  bears  with  it  certain  hereditary  characteristics,  doubtless 
in  the  nucleus  for  the  most  part  ;  (/')  it  brings  with  it  a  stimulus  to  division 
of  a  qualitative  character,  doubtless  in  some  part  in  its  small  cell- 
substance.  This  last  function — the  dynamic  function— Weismann  wholly 
denies.  The  sperm  has  to  him  only  a  quantitative  function.  Yet  in  spite 
of  this  virtual  denial  of  sex, — i.e.^  of  any  deep  difference  between  male 
and  female  whether  elements  or  organisms, — he  does  admit  a  qualitative 
action  after  all,  for  it  is  out  of  the  mingling  of  the  male  and  female  germ- 
plasma  that  all  variations  arise. 

(4.)  Boveri  makes  an  interesting  note  in  regard  to  Weismann's  discovery 
and  theory.  There  is  a  tendency,  illustrated  in  ascarids,  for  the  second 
polar  division  to  limit  itself  to  the  chromatin  elements,  to  be  a  nuclear 
division  rather  than  a  genuine  cell-budding.  Such  a  second  division  may 
possibly  occur  in  the  parthenogenetic  ova,  while  there  may  be  in  reality 
one  extrusion.  A  second  nucleus  may  be  formed,  and  retained,  and  act 
the  part  of  a  spermatozoon,  very  much  as  Minot's  theory  supposes. 

{g)  Our  theory  of  parthenogenesis  is  not  so  subtle  as 
Weismann's  nor  so  simple  as  Minot's.  Just  as  the  spores 
which  illustrate  the  beginnings  of  sex  may  sometimes  dispense 
with  conjugation  and  germinate  independently,  so  may  ova 
develop  parthenogenetically.  These  are  to  be  regarded  as 
incompletely  differentiated  female  cells,  which  retain  a  measure 
of  katabolic  (relatively  male)  products,  and  thus  do  not  need 
fertilisation.     Such  a  successful  balance  between  anabolism  and 


1 84 


THE    EVOLUTION    OF    SEX. 


katabolism  is  indeed  the  ideal  of  all  organic  life.  That  the 
extrusion  of  one  polar  globule  still  occurs,  only  shows  that 
some  katabolic  products  are  still  expelled.  In  parasitic  fungi, 
sexual  reproduction  disappears,  and  surrounding  waste  products 
presumably  help  the  purpose  otherwise  effected  by  sexual 
organs,  so  peculiarities  in  the  conditions  of  parthenogenetic  ova 
may  explain  the  retention  of  the  normal  balance  which  makes 
division  possible  without  the  usual  stimulus  of  fertilisation. 
Abundant  and  at  the  same  time  stimulating  nutrition  (Rolph), 
early  differentiation  of  the  sex-cells  (Simon),  the  general  pre- 
ponderance of  reproductive  over  vegetative  constitution  (Hen- 
sen),  their  liberation  before  the  anabolic  bias  has  carried  them 
too  far,  are  among  these  favouring  conditions.     The  incipient 


(  disease  (u) . 
Female  -,  sex  (s). 

(  parthenogenesis  (p). 


{  parthenogenesis  (p). 
Male      -  sex  (s). 

(  disease  (u). 


Diagram  illustrating  ilie  theory  of  parthenogenesis. 

segmentation  observed  in  a  few  ova  is  an  independent  effort  to 
save  themselves  from  being  too  big  to  live,  since  they  are  not 
passive  enough  to  remain  dormant.  Waste  has  set  in,  self- 
digestion  begins,  the  cell  is  forced  into  the  expedient  of  division. 
In  higher  animals  this  is  all  in  vain  :  in  lower  animals  such  im- 
perfectly  differentiated  female  cells  are  commoner;  they  form 
the  parthenogenetic  ova. 

i:^  9.  Ori<^in  of  Parthenogenesis. — From  the  occurrence  of 
parthenogenesis  in  the  animal  series,  it  is  certain  that  it  has 
originated  as  a  degeneration  from  the  ordinary  sexual  process. 
It  is  no  direct  persistence  of  a  primitive  ideal  state,  though  to 
some  degree  a  recapitulation  of  it.  One  hypothetical  mode  of 
origin,  which  may  well  apply  to  the  rotifers,  is  easily  sketched. 


DEGENERATE  SEXUAL  REPRODUCTION.  I 85 

In  conditions  favouring  katabolism  the  males  wore  themselves 
out,  the  females  became  katabolic  enough  to  do  without  them. 
We  find  the  males,  where  they  persist,  much  smaller  than  the 
female  rotifers,  often  extremely  degenerate,  in  one  section 
wholly  unknown.  Again,  from  the  fact  that  the  interruption  of 
a  parthenogenetic  series  of  females  by  the  appearance  of  males 
usually  occurs  in  hard  times,  we  may  infer  that  prosperous  vital 
conditions  induced  parthenogenesis.  Why  then  are  not  internal 
parasites  parthenogenetic  ?  They  are  very  generally  herma- 
phrodites, and  have  moreover  gone  beyond  parthenogenesis 
to  prolific  asexual  multiplication. 

It  is  misleading  to  interpret  the  occurrence  of  partheno- 
genesis as  due  to  "motives"  and  "important  advantages." 
These  are  afterthoughts  of  our  importation.  It  is  not  easy 
indeed  to  keep  from  metaphorical  language  which  suggests 
that  polar  globule-formation  is  a  "  contrivance,"  and  partheno- 
genesis a  "  device."  Such  casual  words  are  of  little  account;  but 
to  say,  as  Weismann  does,  "  that  sexual  reproduction  has  here 
been  given  up,  not  by  any  chance  nor  from  internal  conditions, 
but  from  quite  definite  external  grounds  of  utility  (Zweck- 
massigkeitsgrunden),"  is  to  say  the  least  misleading.  A  species 
of  crustacean  is  being  decimated  by  enemies,  increased  multi- 
plication would  lessen  the  danger  of  extinction,  parthenogenesis 
is  establised,  and  for  every  one  before  producing  eggs  there  are 
now  two^voi/a  tout.  Against  this  short  and  easy  method  with 
nature  we  emphatically  protest,  and  maintain  that  the  origin  of 
parthenogenesis  was  not  for  any  subsequent  advantage,  but 
purely  from  necessary  internal  conditions. 

§  10.  The  Case  of  Bees.— Wq  have  already  spoken  of  the  "voluntary 
parthenogenesis"  of  bees.  All  the  eggs  are  supposed  to  have  the  power  of 
parthenogenesis,  but  all  are  not  allowed  so  to  develop.  The  fertilised  eggs 
develop  into  queens  and  workers,  the  unfertilised  give  rise  to  drones. 
Weismann  emphasises  the  fact  that  the  ova  are  all  alike.  "There  is  no 
difference  between  those  which  are,  and  are  not  to  be  fertilised.  The 
difference  first  appears  after  the  maturation  of  the  egg,  and  the  removal  of 
the  ovogenetic  plasma."  The  state  of  the  polar  bodies  is  not  known,  so 
the  question  need  not  be  complicated  by  suppositions  about  them.*  Writing 
before  his  discovery  in  regard  to  parthenogenesis,  he  says  the  sine  qua  non 
of  development  is  that  the  nucleus  acquire  a  certain  quantity  of  germ-plasma  ; 
the  fertilised  ^'gg  gets  its  quantum  in  the  usual  way  by  aid  of  the  sperm,  the 
unfertilised  gets^  it  by  simple  growth  ;  the  difference  of  sex  in  the  result 
need  not  be  further  taken  into  account.  Again  we  remark,  that  this  matter 
of  a  quantum  of  "  germ-plasma,"  and  the  two  ways  of  getting  it,  is  a  pure 

*  See,  however,  p.  180,  note. 


1 86  THE    EVOLUTION    OF    SEX. 

supposition,  both  in  general  and  in  this  particular  case.  Again  we  must 
note,  that  if  parthenogenesis  be  decided  on  utilitarian  principles,  and  if  the 
difference  of  sex  need  not  be  taken  into  account,  and  if  the  eggs  are  all  the 
same  to  start  with,  we  see  some  difficulty  in  understanding  the  persistence 
of  drones  and  sexual  reproduction  at  all.  It  is  a  laborious  and  expensive 
way  of  attaining  no  obvious  gain.  But  we  should,  indeed,  like  to  be  sure 
that  the  ova  are  all  the  same  to  start  with.  Von  Siebold  said  that  the 
queen  was  moved  by  the  sight  of  the  different  size  of  the  cells  to  fertilise 
or  refrain  from  fertilising.  This  may  be  so.  Impressive  as  a  queen's  cell 
is,  the  difference  between  a  worker's  and  drone's  is  much  less  striking.  We 
suspect  the  impulse  lies  somewhere  else.  But  barring  this,  the  eggs  laid 
first,  when  the  queen  is  at  its  prime,  develop  into  females  ;  the  eggs  which 
give  rise  to  drones  come  later,  when  the  mother  is  more  exhausted.  They 
have  had  less  chance  of  differentiation — they  are  parthenogenetic  ova.  So 
with  old  queens,  when  the  stock  of  sperms  is  also  of  course  exhausted. 
Weismann  quotes  the  experiment  which  Bessels  made,  after  Dzierzon.  The 
nuptial  flight  was  prevented,  and  ova  which,  in  the  course  of  nature,  would 
have  been  fertilised  and  given  rise  to  queens  and  workers,  were  of  course 
unfertilised,  and  developed  parthenogenetically  into  males.  This  proves, 
he  says,  that  the  ova  are  all  the  same  to  start  with.  But  one  would  like 
to  know  whether  the  prevention  of  the  nuptial  flight  had  not  also  its  effect 
upon  the  ova,  and  whether  the  parthenogenetic  ova  are  not  always  less 
differentiated. 


DEGENERATE    SEXUAL    REPRODUCTION.  1 87 


SUMMARY. 

1.  Parthenogenesis  was  formerly  believed  to  be  of  wider  occurrence 
than  it  really  is,  but  it  is  definitely  known  to  be  not  uncommon  in  lower 
animals. 

2.  Artificial,  pathological,  occasional,  partial,  seasonal,  juvenile,  and 
total  parthenogenesis  must  for  clearness  be  distinguished. 

3.  The  occurrence  of  parthenogenesis  is  especially  well  seen  in  rotifers, 
crustaceans,  and  insects. 

4.  It  is  rare  among  plants,  but  certainly  occurs  in  some  of  the  lower 
forms. 

5.  The  offspring  of  parthenogenetic  ova  is  very  diverse. 

6.  The  effects  of  parthenogenesis  on  the  species  deserve  consideration, 
especially  by  those  who  find  in  sexual  intermingling  the  sole  fountain  of 
specific  variation. 

7.  Parthenogenetic  ova,  so  far  as  observed,  form  only  one  polar  body, 

8.  Parthenogenetic  ova  are  here  regarded  as  imperfectly  differentiated 
female  cells,  retaining  certain  male  or  katabolic  characteristics. 

9.  In  origin  parthenogenesis  is  regarded  as  a  degeneration  from  the 
ordinary  sexual  process. 

10.  The  voluntary  parthenogenesis  of  bees  is  taken  as  a  concrete 
illustration. 

LITERATURE. 

See  especially  the  already  cited  works  of  Balfour,    Brooks,    Hensen, 
Minot,  Rolph,  Sachs,  Weismann  ;  also — 
Owen.— Parthenogenesis  ;  or.  The  Successive  Production  of  Procreating 

Individuals  from  a  Single  Ovum.     London,  1849. 
Von  SiEBOLD. — Beitrage  zur  Parthenogenesis.     Leipzig,  1871. 
Leuckart. — Art"Zeugung"  in  Wagner's  Handworterbuch  d.  Physiol., 

Bd.  IV.,  1853. 
Gerst.«cker. — Bronn's  Klassen  und  Ordnungen  des  Thierreich,  \ol.  V., 

Arthropoda. 
Brooks,  W.  K. — Law  of  Heredity.     Baltimore,  1883. 
Simon,  F. — Die  Sexualitat,  &c.,  Inaug.  Dissertation.     Breslau,  1883. 
Blochmann — Ueber  die  Richtungskorper  bei  Insekteneiern,  Biolog.  Cen- 

tralblatt,  VII.,  and  Morpholog.  Jahrbuch,  XII. 
Weismann,  A. — Beitr.  zur  Naturgeschichte  der  Daphnoiden.     Leipzig, 

1876-79.        Ueber  die  Zahl  der  Richtungskorper  und  iiber  ihre  Be- 

deutung  fiir  die  Vererbung.     Jena,  1887. 
Weismann,  A.,  and  Ischikawa,  C. — Berichten  der  naturforsch.  Gesell- 

schft.,  Freiburg,  III.,  1887. 
Hudson  and  Gosse. — The  Rotifera.     London,  1886. 
Plate. — Beitrage  zur  Naturgeschichte  der  Rotatorien,  Jenaische  Zeitschft. 

f.  Naturwiss,  XIX.,  1886. 
Karsten,  H. — Parthenogenesis  und  Generations-Wechsel   im   Thier  und 

Pflanzenreiche.     Berlin,  1888. 


CHAPTER   XIV. 

Asexual  Reproduction. 


§  I.  Artificial  Division. — Weeping  willows  are  by  no  means 
scarce  trees  in  Britain,  yet  as  they  never  flower,  they  must  all  have 
grown  from  slips,  or  in  other  words  artificial  asexual  multiplica- 
tion. So  too,  only  more  naturally,  the  Canadian  pond-weed  has 
spread  prodigiously   in    our   lochs,    canals,    and   rivers,  never 


A  group  of  Sea-Anemones.  —  From  Andres, 


flowering,  but  owing  its  increase  wholly  to  the  asexual  process. 
Every  one  knows  how  the  gardener  increases  his  stock  by  slips 
and  cuttings,  thus  taking  advantage  of  the  power  a  part  has  to 
reproduce  the  whole.  Quite  in  the  same  way,  cultivators  of 
bath  sponges  bed  out  little  fragments  to  keep  up  a  convenient 


ASEXUAL    REPRODUCTION. 


189 


supply.  In  the  last  century,  the  Abbe  Trembley  delighted 
himself  and  others  by  the  often  repeated  observation,  that  to 
get  many  hydra  polypes  out  of  one,  the  simplest  and  quickest 
way  was  to  cut  it  in  pieces.  Though  the  fragment  be  very 
small,  it  will  reproduce  the  whole,  provided  always  that  it  have 
to  start  with  fair  samples  of  the  different  kinds  of  cells  in  the 
body.  The  same  may  be  done  any  day  with  the  much  larger 
sea-anemones.  So  the  earthworm,  curtailed  by  the  spade,  does 
not  necessarily  suffer  loss,  though  it  suffer  pain.  The  head  por- 
tion grows  a  new  tail,  and  even  a  decapitated  portion  may 
reproduce  a  head  and  brain,  not  that  this  is  saying  much  for 
these. 

§  2.  Regeneration.  —  Spades  and  knives  are  not  exactly 
instruments  of  nature,  but  they  have  their  counterparts.  Fight- 
ing with  a  rival  a  crab   may  lose  its  claw,  or  the  same  may 


The  Formation  of  a  Sponge  Colony  {Olynthtis)  by 
budding. — After  Haeckel. 

happen  in  the  frequently  fatal  moulting,  which  seems  almost 
like  a  mistake  in  nature.  Slowly,  however,  forgiving  nature 
makes  good  the  loss ;  the  cells  of  the  stump  multiply,  and 
arrange  themselves  in  obedience  to  the  same  necessities  as 
before,  and  a  limb  is  regenerated.  Many  an  appendage  among 
the  lower  animals  is  from  time  to  time  nipped  off,  only  to  be 
grown  again.  A  snail  has  been  known  patiently  to  regenerate 
an  amputated  eye-bearing  horn  twenty  times  running.  Sometimes 
one  is  tempted  to  think  that  the  animals  almost  understand  that 
it  is  better  for  one  member  to  perish  than  for  the  whole  life  to  be 
lost,  so  readily  does  a  starfish  surrender  an  arm,  or  a  lizard  its 
tail.  Yet  it  must  be  recognised  that  animals,  like  men,  are  often 
wiser  than  they  wot  of.  In  the  panic  of  capture,  strong  con- 
vulsions   may  occur,   which  surprise   and   perhaps   shock  the 


190  THE    EVOLUTION    OF    SEX. 

molester  of  a  sea-cucumber  by  the  ejection  of  its  viscera ;  or  a 
tetanic  contraction  of  the  muscles  makes  the  slow-worm  brittle 
in  the  hands  of  its  captor.  The  power  of  regeneration  is  most 
marked  in  echinoderms,  but  persists  as  high  up  as  reptiles. 
The  regrowth  of  part  of  a  lizard's  leg  is  the  chef-d'csuvrc  in  this 
line.  Beyond  that,  regeneration  is  restricted  to  little  things. 
We  constantly  regenerate  the  skin  of  our  lips,  but  we  cannot 
naturally  replace  an  amputated  limb.  It  is  more  marvellous 
that  we  cannot,  than  that  the  lizard  can.  That  the  cells  of  an 
irritated  stump  should  divide  and  multiply,  and  that  the  result 
should  be  the  same  as  it  was  at  the  first,  is  really  no  marvel,  or 
rather  as  much  as,  but  no  more  than  the  original  development. 
The  dividing  cells  of  the  growing  stump  are  simply  repeating 
their  original  development. 

§  3.  Degrees  of  Asexual  Reprod2iction. — The  keynote  of  the 
subject  was  truly  struck  by  Spencer  and  Hseckel,  when  they 
defined  asexual  reproduction  as  discontinuous  growth.  All 
growth  is  a  reproduction  of  the  protoplasm  and  its  nuclear 
elements,  or  in  short  of  the  cells ;  all  reproduction  (excluding 
the  important  fact  of  fertilisation)  is  growth.  The  ovum, 
asexually  produced  from  the  parent  ovum  or  its  lineal  de- 
scendant cells,  grows  and  reproduces  itself  in  turn,  building  up 
the  embryo.  The  embryo  grows  into  an  adult  organism,  and 
the  surplus  of  continued  growing  energy  results  in  the  asexual 
production  of  buds,  or  the  sexual  discharge  of  differentiated 
reproductive  elements.  We  start  from  the  ordinary  processes 
of  cell-multiplication  and  regeneration  exhibited  in  the  normal 
organism.  Then  come  the  processes  by  which  lost  members 
are  regenerated,  involving  more  or  less  serious  extra  growth. 
To  these  we  must  add  the  rarer  and  yet  not  rare  cases,  where 
the  artificial  halves  or  fractions  of  an  organism  can  grow  into 
wholes.  Normal  and  frequent  however  are  the  very  abundant 
cases  of  budding,  where  a  sponge  or  hydra,  zoophyte  or  coral, 
has  surplus  enough  to  grow  off  new  individuals,  which  remain 
continuous  with  itself.  The  parent  organism,  whether  zoophyte 
or  strawberry-plant,  has  an  asexually  produced  progeny  round 
about,  and  in  asexual  continuity  with  itself.  But  they  do  not 
always  remain  continuous ;  the  hydra  produces  buds,  but 
eventually  sets  them  adrift.  This  is  still  better  seen  in  many 
of  the  hydroids,  where  individuals  are  separated  off  as  swim- 
ming-bells or  medusoids.  The  multii)lication  has  become 
discontinuous.     Continue  the  process,  and  we  find  the  libera- 


ASEXUAL    REPRODUCTION. 


191 


tion  of  special  cells,  clinging  often  for  a  time  to  the  parent, 
generally  dependent  for  development  on  union  with  similar 
cells  of  complementary  constitution  ;  we  find,  in  fact,  the  sexual 
reproduction  which,  in  the  higher  organisms,  so  thoroughly 
replaces  the  asexual  process. 

§  4.  Occurrence  of  Asexual  Rep7'odjiction  in  Plants  and 
Animals. — In  plants,  as  one  would  expect  from  their  typical 
vegetative  constitution,  the  asexual  process  is  common,  particu- 
larly among  the  lower  forms.  The  most  familiar  of  all  cases  is 
afforded  by  the  common  liverworts  {Marchantia  and  Lunula7'ia)^ 


Asexual  Propagation  of  Grass — {a)  the  bulbils 
rooting  on  the  ground  ;  {h)  their  appear- 
ance in  the  inflorescence ;  {c)  a  small 
portion  enlarged. — From  nature. 

which  through  the  formation  of  asexual  buds  or  gemmae  in  the 
cups  so  familiar  upon  their  thallus,  are  enabled  to  overrun  our 
flower-pots,  and  so  rapidly  become  a  pest  of  the  greenhouse. 
Many  ferns  too,  notably  among  the  Aspleniums,  reproduce  by 
bulbils,  arising  upon  the  frond;  and  the  bulbils  which  arise  in  the 
axils  of  the  leaves  of  the  tiger-lily  are  familiar  missiles  for  every 
child  accustomed  to  a  flower-garden  (see  figs.  pp.  226  and  287). 
The  alliums,  and  some  of  our  common  grasses  also,  furnish  us 
with  examples  of  the  replacement  of  flowers  by  separable  buds. 


192  THE    EVOLUTION    OF    SEX. 

Asexual  reproduction  or  multiplication  by  more  or  less  dis- 
continuous growth,  without  the  differentiation  of  special  and 
mutually  dependent  sex-cells,  occurs  from  the  simplest  animals 
on  to  the  tunicates  or  sea-squirts,  from  the  base  to  just  over  the 
line  which  separates  backboneless  and  backboned  animals.  It 
is  necessary,  however,  to  review  the  groups. 

Protozoa. — Fertilisation  began  in  almost  mechanical  fusion.  Reproduc- 
tion begins  with  almost  mechanical  rupture.  The  unit  mass  of  protoplasm, 
becoming  too  big  for  control,  breaks.  Thus  it  saves  itself,  and  at  the  same 
time  multiplies.  Such  breakage  may  be  seen  in  a  primitive  form  like 
Sthizogenes,  but  it  also  occurs  in  a  few  of  the  relatively  high  infusorians. 
That  the  breakage  sometimes  means  dissolution  is  certain  ;  nor  is  reproduc- 
tion ever  so  very  far  removed  from  death. 

The  rupture  becomes  orderly  and  systematic  in  budding.  This  may  be 
multiple,  as  in  the  common  Arcella,  where  a  number  of  small  buds  are 
constricted  off  all  round.  But  the  process  is  oftener  concentrated  in  one 
extrusion  or  overflow.  In  budding,  the  separated  daughter-cell  is  in 
varying  degree  smaller  than  the  parent,  and  the  process  resembles  an  over- 
flow. When  the  bud  is  approximately  equal  to  the  parent,  and  the  process 
is  of  the  nature  of  a  constriction,  it  is  of  course  division. 

The  division  may  also  be  multiple,  taking  place  in  rapid  succession  and 
in  limited  space,  e.g.,  within  a  cyst.  Then  we  speak  of  spore-formation. 
The  last  three  modes  of  multiplication  are  exceedingly  common  among 
Protozoa. 

These  buddings  and  divisions  are  not  of  course  rough  and  ready 
processes.  The  nucleus  almost  always  shares  in  them  in  an  orderly  and 
deliberative  fashion.  There  are  variations  in  its  behaviour  as  in  higher 
animals,  but  there  is  no  doubt  that  cell-division,  with  a  gradient  of  progress 
like  everything  else,  is  essentially  one  and  the  same  in  the  vast  majority  of 
cases.  Gruber  has  been  especially  successful  in  proving  that  fragments  of 
Protozoa,  artificially  separated  without  nuclear  elements,  cannot  live  long, 
though  they  may  grow  and  repair  their  losses  for  a  little.  The  nucleus  is 
essential  to  life,  though  sometimes  it  seems  to  disappear,  and  become  as  it 
were  a  diffuse  precipitate  in  the  protoplasm. 

Sponges. — In  sponges  no  one  can  fail  to  recognise  the  impossibility  of 
drawing  any  rigid  line  between  growth  and  asexual  reproduction.  Between 
simple  extension  of  the  parent  mass,  and  the  budding  off  of  new  individuals, 
no  sure  distinction  can  in  many  cases  be  made  out.  Sponges  do  not  divide, 
though  they  may  be  cut  up,  yet  they  give  off  discontinuous  buds.  An  out- 
grown tube  may  lose  connection  with  the  parent,  or  a  great  tumour-like 
mass  may  be  slowly  extruded,  or  tiny  brood-buds  may  be  set  adrift  to  shift 
for  themselves.  In  disadvantageous  conditions  the  surface  of  a  sponge 
sometimes  gathers  into  minute  superficial  buds,  by  means  of  which  it  is 
possible  that  the  life  is  saved. 

In  the  fresh-water  sponges,  in  disadvantageous  circumstances, — of  cold 
in  some  countries,  heat  and  drought  in  others, — some  of  the  cells  club 
together  to  form  gemnmles,  which  often  save  the  life  of  the  otherwise  dying 
sponge.  They  are  complex  enough,  with  sheaths  and  spicules,  and  some- 
times even  with  a  float,  but  in  principle  they  simply  do  by  a  multiple  union 
what    is   otherwise  attained    by  ovum  and  sperm.     Best    known  in   this 


ASEXUAL    REPRODUCTION.  1 93 

respect  is  the  freshwater  sponges  {Spongilla)  ;  they  have  also  been  described 
in  other  common  sponges,  e.g.^  in  Ch'oue,  the  borer  in  oyster  shells. 

Ccclenteratcs. — In  such  names  as  zoophytes,  sea-firs,  sea-roses,  there 
is  a  prevision  of  the  undoubtedly  plant-like  character  of  many  of  the 
coelenterates.  A  sessile  habit  is  very  general,  though  often  only  a  phase 
in  the  life-history,  and  asexual  reproduction  runs  riot.  A  well-fed  hydra 
is  prolific  in  bud-bearing  ;  and  numerous  gradations  connect  this  with  the 
myriad  colonies  exhibited  by  many  hydroids.  The  individuals  forming  a 
united  family  share  in  the  common  life  and  nutriment.  As  the  colony 
becomes  complex,  it  is  often  physically  impossible  for  all  the  members  to 
remain  on  terms  of  even  approximate  equality  of  internal  and  external 
conditions.  One  becomes  relatively  overfed,  another  starved.  Slight 
differences  of  function  gradually  become  emphasised  and  exaggerated,  till 
division  of  labour  is  established.  The  structural  aspect  of  this  is  differen- 
tiation or  polymorphism  among  the  members  of  the  colony,  and  results  in 
the  establishment  of  nutritive  and  reproductive,  sensitive  and  protective, 
"  persons."     Thus  in  the  common  Hydractinia,  the  open-mouthed  nutritive 


One  of  the  acarids  or  lice  (jGiyci^has^'ies  ciirsor)  forming  a  life-saving  cyst,  while  the 

individual  itself  dies. 

individuals  are  markedly  contrasted  with  the  dependent  reproductive 
persons ;  and  again,  in  different  form,  the  rhythm  repeats  itself  in  the  contrast 
between  active,  offensive,  and  sensitive  elongated  members,  and  entirely 
passive  and  abortive  spines,  which  form  a  chevaux-de-frise  under  shelter  of 
which  the  others  cower.  It  is  usually  supposed  that  the  sessile  hydroids 
are  in  a  sense  degenerate  from  more  active  ancestral  types.  The  free- 
swimming  embryo  becomes  exhausted,  settles  down,  and  exhibits  pre- 
dominant vegetativeness  with  postponed  sexuality.  In  many  cases, 
however,  there  is  a  recovery  of  the  ancestral  liberty  of  action,  for  modified 
"  persons  "  are  set  adrift  as  active,  free-swimming,  sexual  medusoids. 

There  are,  however,  active  forms  of  the  true  medusoid  type  (  7?-a<r/y/- 
inedus(F)  which  never  descend  to  the  sessile  nadir  of  existence,  but  yet 
exhibit  the  asexual  tendency  of  the  class  in  forming  temporary  clusters  of 
pendent  buds.  Lang  has  lately  described  a  remarkable  compound 
medusoid  {Gastroblasta  raffaelii),  which  has  sometimes  as  many  as  nine 
stomachs,  and  may  be  assumed  to  be  highly  nutritive.  The  remarkable 
point,  however,  is  that  the  compound  adult  is  the  result  not  only  of 
continued  budding,  but  of  a  process  of  rectangular  incomplete  division. 
Along  with  some  others  it  leads  on  towards  the  Portuguese  man-of-war,  or 

N 


194 


THE    EVOLUTION    OF    SEX. 


siphonophore  series.  Here  the  larva  develops  at  first  into  a  simple 
medusa-like  individual,  but  this  buds  off  a  manifold  series  of  "  persons," 
which,  by  dislocation  or  even  migration,  become  arranged  in  all  the  beauty 
of  the  siphonophore  colonies,  which  surpass  even  Hydractinia  in  their 
division  of  labour.  It  is  difficult  enough  in  some  cases  to  distinguish 
between  true  "persons" — which  Hreckel  calls  "  Medusomes" — and  mere 
organs  like  protective  bracts,  which  are  also  budded  off. 


Siphonophore  Colony,  showing  the  float  (n),  the 
swimming-bells  {d),  and  the  nutritive,  repro- 
ductive, and  other  "persons",  beneath. — 
From  Lang,  after  Ha;ckel. 


In  another  direction,  viz.,  among  the  true  jelly-fishes  [Aa-aspeda), 
where  an  active  habit  greatly  preponderates,  we  still  find  the  occurrence  of 
asexual  multiplication.  Some  forms  [e.g.^  Pc/agm)  are  entirely  free;  at  the 
opposite  extreme  a  few  {^Lticernarida)  may  be  described  as  sedentary  ; 
between  these  we  find  the  common  aurelia,  which  settles  down  in  its  youth, 
and  gives  rise  by  division  to  what  afterwards  become  the  large  sexual 
jelly-fishes  (see  fig.  p.  202). 

There  remains  two  classes  of  coelenterates, — the  Ctenophora,  like  Beroe, 
which  represent  a  climax  of  activity,  and  never  divide;  and  the  Actinozoa 


ASEXUAL    REPRODUCTION. 


195 


(sea-anemones  and  corals),  which  lead  to  a  passive  terminus  again,  and 
exhibit  profuse  asexual  multiplication.  A  few  sea-anemones  divide 
normally,  just  as  they  may  be  multiplied  by  artificial  cutting.  Fragments 
may  also  be  given  off  in  an  arbitrary  sort  of  fashion,  reminding  one  of  the 
overflow  buds  of  sponges.  The  division  may  be  either  longitudinal  or 
crosswise  in  sea-anemones,  and  the  budding  of  corals  takes  many  forms, 
resulting  in  the  quaint  complexity  of  brain-corals  and  the  like.  In  one  sea- 
anemone  {Gonactinia  prolifera)^  where  transverse  division  occurs,  it  is 
interesting  to  notice  that  this  has  only  been  observed  in  young  forms  with 
undeveloped  sexual  organs.  It  recalled,  in  fact,  the  asexual  multipliaticon 
of  a  young  jelly-fish.  In  another  of  the  corals 
{Antipatharia)  Brook  has  recently  observed  how  a 
nutritive  "person"  may  by  constriction  form  a  re- 
productive individual  on  either  side. 

Worms. — The  lower  worm-types  are  roughly  dis- 
tinguishable from  most  of  the  higher  by  the  broad  fact 
that  they  are  all  of  a  piece,  without  rings  or  segments. 
A  physiological  link,  however,  between  worms  of  only 
one  segment  and  those  with  many,  is  found  in  the 
asexual  chains  which  some  of  the  former  occasionally 
develop.  Thus  the  little  turbellarian  Microstoimim 
lineare  may  bud  off  a  temporary  chain  of  sixteen  in- 
dividual links.  The  budding  begins  at  the  posterior 
end,  and  what  is  partly  separated  off  is  a  portion  in 
excess  of  the  normal  size.  The  second  link  grows 
till  it  attains  the  usual  adult  size,  and  as  it  exceeds 
this  form  a  third  link.  At  the  same  time  the  original 
individual  may  also  be  doing  the  same,  and  thus  a 
chain  of  four  is  formed.  Two  more  buddings  by  each 
of  the  links  bring  the  asexual  process  to  a  climax,  and 
then  the  individuals  separate  from  one  another  and 
become  sexual  in  freedom.  It  is  important  to  notice 
that  the  asexual  reproduction  takes  place  in  favourable 
nutritive  conditions,  and  as  each  individual  exceeds 
its  normal  limit  of  growth.  In  some  allied  planarians 
the  asexual  multiplication  is  effected  not  by  budding 
but  by  division.  Zacharias  observed,  that  when  nutri- 
tion was  checked  the  vegetative  increase  ceased,  and 
sexual  reproduction  set  in.  Not  quite  parallel  with  the 
above,  but  quite  asexual,  is  the  prolific  multiplication 
characteristic   of  the   flukes   and    tapeworms.      The  "'^• 

common  liver-fluke  has  often  several  asexual  generations  before  it  finds  its 
final  host  in  the  sheep,  and  is  surpassed  in  this  respect  by  some  of  its 
relatives.  The  bladder-worm,  in  passive  ease,  with  a  plethora  of  nutrition, 
may  form  asexually  many  "heads,"  each  of  which,  inside  a  future  host, 
grows  out  into  the  long  series  of  joints  which  compose  the  tapeworm.  In 
their  profuse  asexual  multiplication  these  parasites  are  like  parasitic  fungi, 
but  unlike  them  in  the  retention  of  the  sexual  process  to  boot. 

In  their  asexual  reproduction,  the  Polyzoa  recall  sponges,  for  not  only 
do  they  all  multiply  by  budding,  and  that  abundantly,  but  they  form 
peculiar  winter-buds  like  sponge-genimules,  by  which  on  the  death  of  the 
parent  the  continuity  of  life  is  nevertheless  sustained.     The  winter-buds  or 


Diagrammatic  repre- 
sentation of  tlie  for- 
mation of  a  chain 
of  individuals  in  the 
Turbellarian  worm 
Microstojtiujii  lin- 
eare.— From    Leu- 


196 


THE    EVOLUTION    OF    SEX. 


statol)lasts    may   further  resemble    sponge-gemmiiles  in   elaborateness   of 
external  equipment,  a  common  characteristic  of  passive  resting  structures. 


A  Sea-worm  {iMyrJanida)  which  has  budded  off  a  chain  of  individuals.— After 

Mihie-Edwards. 


In  the  higher  bristle-footed  worm-types  (C/z^r/^/^^/a),  asexual  multiplica- 
tion occurs  in  great  variety  of  expression.     Some,  when  alarmed,  break  up 


Syllis  rnjnosa,  a  ringed  marine  worm,  in  which  asexual  multiplication  has  pro- 
duced a  branched  appearance. — From  M'lntosh,  "  Challenger  "  Rep.  on 
Annelida. 


ASEXUAL    KEPRODUC'lION. 


197 


in  a  panic,  but  a  few  are  also  known  to  do  this  in  apparently  normal  life. 
Each  part — there  may  be  more  than  two — reproduces  the  whole.  Thus,  at 
a  comparatively  high  level  among  animals,  reproduction  may  be  literally 
rupture.  Oftener,  however,  budding  precedes  the  division,  and  curious 
chains  of  ringed  worms  are  thus  produced.  Nor  do  the  budded  individuals 
always  keep  in  a  straight  line,  but,  as  in  the  freshwater  naids,  may  abut  at 
angles,  and  form  a  quaint  living  branch.  To  what  degrees  this  irregularity 
of  budding  may  attain  is  well  seen  in  the  accompanying  cut  of  a  portion  of 
a  worm  {Syllis  ra//iosa),  found  on  the  "  Challenger  "  voyage.  The  buds 
occur  laterally,  terminally,  or  on  any  broken  surface,  and  the  result  is  an 
almost  bush-like  compound  organism  rivalling  even  the  hydroids  in  the 
freedom  of  its  branching.  .Some  of  the  branches  become  males  or  females,  and 
go  separate,  or  are  sent  adrift.     In  other  syllids,  the  separation  of  a  series 


Comet  form  of  a  Starfish,  showing  how  one  arm  "  regenerates"  or  reproduces 
otlier  four. — From  Carus  Sterne,  after  Ha^ckel. 

of  joints  as  a  sexual  individual  has  been  repeatedly  observed,  or  this  may 
be  reduced  till  only  one  joint,  laden  with  reproductive  elements,  is  set  free. 
In  many  of  these  chxtopods  the  budding  begins  when  the  normal  size  of 
the  individual  has  been  stopped  by  unfavourable  conditions,  which  bring 
about  separation,  and  the  sul;sequent  sexuality  of  the  liberated  individuals. 


Adventitious  buds  forming  at  the  sides  of  a  \^vS  oi  BryopJiylluju 
calycimuii. —  From  nature. 

Starfishes  and  the  like  surrender  their  "arms"  so  readily,  that  some 
have  supposed  that  they  might,  in  this  way,  normally  multiply.  A 
voluntary  surrender  of  parts  as  a  mode  of  multiplication  is,  however, 
in  this  case  difficult  to  prove.  So  while  crustaceans,  insects,  spiders, 
and  molluscs  may  lose  and  regrow  certain  parts,  no  asexual  multiplication 
occurs. 


198  THE    EVOLUTION    OF    SEX. 

In  the  tunicates  the  asexual  process  has  again  full  play.  It  is  not 
confined  to  the  passive  sessile  forms,  where  one  might  expect  it,  but  occurs 
in  some  of  the  free-swimmers  as  well.  From  a  creeping  stem  buds  may 
arise,  like  plants  from  a  rhizome  ;  or  a  parent  form  may  bud  oft"  all  round, 
and  finally  die  away,  leaving  the  offspring  in  a  circle  round  a  cavity.  Both 
by  budding  and  division  chains  may  be  formed,  as  in  the  salpas.  In  these 
lowly  vertebrates  asexual  multiplication  terminates.  How  the  process 
often  alternates  in  regular  rhythm  with  ordinary  sexual  reproduction  will 
be  discussed  in  the  next  chapter. 


ASEXUAL    REPRODUCTION.  1 99 


SUMMARY. 

1.  Artificial  division  may  be  readily  utilised  as  a  means  of  multiplica- 
tion in  plants  and  in  lower  animals. 

2.  Regeneration   of  lost   parts  is   very  common  both   in    plants   and 
animals. 

3.  Asexual    reproduction   from    continuous    budding    to    discontinuous 
multiplication  has  many  degrees,  leading  on  to  the  sexual  process. 

4.  It  occurs  throughout  the  series  from  Protozoa  to  Tunicata. 


LITERATURE. 

General  Works  cited  ;  the  ordinary  Zoological  and  Botanical  Text-books  ; 

and, 
Lang,  A. — Der  Einfluss  des  Festsitzen  auf  den  Thieren,  und  der  Ursprung 

der  ungeschlechtlichen  Fortpflanzung.     Jena,  1886. 
Spencer. — Principles  of  Biology,     London,  1866. 
H/ECKEL. — Generelle  Morphologic.     Berlin,  1866. 
Freuericq. — La  Lutte  pour  I'Existence  chez  les  Animaux  INIarins.     Paris, 

1889.     (For  "  Regeneration  of  Parts,"  &c. ) 


CHAP'l'ER  XV. 

Alternation  of  Generations. 

§  I.  History  of  Discovery. — Early  in  the  century  the  poet 
Chamisso,  accompanying  Kotzebue  on  his  circumnavigation  of 
the  globe,  observed  in  one  of  the  locomotor  tunicates  {Sal pa) 
that  a  solitary  form  gave  birth  to  embryos  of  a  different  char- 
acter, connected  together  in  chains,  and  that  each  link  of  the 
chain  again  produced  a  solitary  form.  Chamisso's  observation 
does  not  seem  to  have  been  quite  accurate,  but  there  is  no 
doubt  that  he  first  called  attention  to  what  is  by  no  means  an 
uncommon  fact,  that  an  organism  produces  an  offspring  very 
unlike  itself,  which  by  and  by  gives  origin  to  a  form  like  the 
parent.  The  progress  of  marine  zoology  and  the  study  of 
parasitic  worms  gave  naturalists  like  Sars,  Dalyell,  Loven,  Von 
Siebold,  and  Leuckart,  early  glimpses  of  many  alternations  in 
life-history,  but  Steenstrup  was  the  first  to  generalise  the  results. 
This  he  did  (1842)  some  twenty  years  after  Chamisso,  in  a 
work  entitled  "  On  the  Alternation  of  Generations ;  or.  The 
Propagation  and  Development  of  Animals  through  Alternate 
Generations,  a  peculiar  form  of  fostering  the  young  in  the 
lower  classes  of  animals."  From  hydroids  and  flukes,  he  gave 
illustrations  of  the  "natural  phenomena  of  an  animal  producing 
an  offspring,  which  at  no  time  resembles  its  parent,  but  which 
itself  brings  forth  a  progeny  that  returns  in  its  form  and  nature 
to  the  parent."  The  interpolated  generation  he  distinguished 
by  the  name  of  "Amme"  or  "wet-nurse."  In  1849,  Owen 
submitted  Steenstrup's  essay  to  stern  criticism,  rejecting  especi- 
ally the  metaphorical  name  "nurse"  as  but  a  verbal  explanation, 
and  proposing  to  explain  what  he  also  called  "alternation 
of  generations,"  along  with  parthenogenesis  and  other  pheno- 
mena, by  the  supposition  of  a  residual  germ  force  or  spermatic 
power  in  the  cells  of  the  apparently  asexual  offspring.  In 
this  he  partially  prophesied  the  modern  conception  of  a 
residual    persistent   germ-plasma.      Soon    afterwards    Leuckart 


ALTERNATION    OF    GENERATIONS. 


!OI 


attempted  to  treat  all  as  cases  of  metamorphosis,  thereby  greatly 
extending  the  meaning  of  that  term.  The  labours  of  some  of 
the  foremost  naturalists  have  both  extended  Steenstrup's  obser- 
vations and  rendered  them  more  precise.  We  now  know  that 
the  phenomenon  is  of  wider  occurrence  than  was  at  first  sup- 
posed, and  also  that  the  title  has  been  unduly  extended  to  cover 


Diagrammatic  representation  of  alternation  of 
generations,  as,  asexual  generation  ;  ,v, 
sexual  generation. 

II.  Shows  alternation  of  asexual  (as) 
and  sexual  (s)  generations. 

In  I.  the  sexual  is  becoming  increas- 
ingly subordinated  to  the  asexual  (as  in 
flowering  plants). 

In  III.  the  asexual  is  increasingly 
subordinated  to  the  sexual  (in  mosses). 

several  entirely  different  sets  of  facts.  It  is  necessary,  therefore, 
to  notice  the  various  forms  which  the  rhythm  of  reproduction 
may  take. 

§  2.  The  Rhythm  between  Sexual  and  Asex2ial  Reproduction. — 
The  clearest  case  to  start  with  is  that  of  many  hydroids. 
A  sessile,  plant-like  zoophyte,  which  buds  off  numerous  nutritive 
persons,  produces  in  the  warm  months  modified  individuals 
which  are  set  adrift  as  medusoid  persons.  Unlike  the  hydroid 
which  bore  them,  these  become  sexual;  and  from  their  fertilised 


202 


THE    EVOLUTION    OF    SEX. 


ova  an  embryo  develops,  which  eventually  settles  down  to  start 
a  new  sessile  colony.  And  thus  through  the  seasons  we  have 
hydroid  asexually  producing  sexual  medusoids,  and  these  again 
producing  hydroids.  The  life-history  for  two  complete  rhythms 
may  be  written  in  the  formula,  in  which  M,  F,  and  A  stand  for 
male,  female,  and  asexual  forms  respectively, — 

^^  -  A  --^^  -  A  ~^^ 
F  F  F 


A,  asexual  hydroid  ;  S,  sexual  medusoid  ;  fertilised  ova  at  base. 

Or  take,  in  slight  contrast,  the  life-story  of  the  common  jelly- 
fish Aurelia.  Large  free-swimming  sexual  animals  produce  ova 
which  are  fertilised  by  sperms ;    the  embryo  develops,  not  how- 


Tlie  alternation  of  generations  in  the  common  jelly-fish  Awelia  ;  i.  the  free-swimming  embryo, 
or  planula  ;  2,  the  embryo  settled  down  ;  3,  4,  5,  6,  the  developing  asexual  stage,  or  hydra- 
tuba  ;  7,  8,  the  formation  of  a  pile  of  individuals;  9,  the  liberation  of  thise  ;  10,  11,  the 
acquisition  of  the  free-living  sexual  medusa  form. — From  Hajckel. 


ALTERNATION    OF    GENERATIONS.  203 

ever  into  a  jelly-fish,  but  into  a  sessile  hydroid-like  organism  or 
"hydra-tuba."  By  growth  and  division  this  asexually  produces 
the  jelly-fish  in  turn.  Here  the  sexual  generation  is  more  stable 
and  conspicuous,  the  reverse  of  the  former  case,  but  the  same 
formula  applies. 

Or  take  a  case  from  another  class  of  animals,  the  marine 
worms.  Some  of  the  syllids  have  the  following  life-history.  A 
worm  remains  asexual,  never  attaining  either  the  external 
characteristics  or  the  internal  organs  of  the  sexual  individuals. 
It  gives  rise  to  these,  however,  by  an  asexual  process  of  chain- 
making.  Sexual  individuals  are  budded  off  from  the  asexual,  into 
which  their  fertilised  ova  in  turn  develop.  This  must,  of  course, 
be  distinguished  from  cases  where  asexual  multiplication  is  only 
a  phase  preceding  the  acquisition  of  sexuality.  The  above 
cases  are  again  expressible  in  the  simplest  formula. 

{b)  Now  take  a  more  complex  case,  from  among  the  tunicates, 
the  highest  point  at  which  the  genuine  alternation  can  be  said 
to  occur.  From  a  fertilised  ovum  in  Salpa,  a  nurse  or  asexual 
individual  develops.  This  has  a  root-like  process  or  stolon,  on 
which  buds  are  formed.  These  are  set  free  together,  and  form  a 
chain  of  sexual  salps.  The  chain  finally  breaks  up.  The  fertilised 
ova  of  the  sexual  salps  grow  up  into  nurses  again.  Now  the 
only  emphatic  complication  here  is  the  liberation  of  a  chain  of 
individuals  at  once  ;  otherwise  the  formula  holds  perfectly  good. 

In  the  allied  Doliolum,  however,  the  case  is  different.  From 
a  fertilised  ovum  a  nurse,  or  asexual  individual,  develops  as 
before.  This  produces  a  number  of  primitive  buds,  which 
cluster  about  the  nurse.  Many  of  them  form  nutritive  in- 
dividuals, and  these  we  may  leave  alone.  But  others  become 
"  foster-mothers,"  and  go  free,  carrying  with  them  a  few  of  the 
primitive  buds, — as  it  were  their  younger  sisters.  The  foster- 
mother  remains  asexual,  is  a  bearer  merely,  and  need  not  further 
complicate  the  series.  But  the  primitive  buds  which  have  been 
carried  away  give  rise  asexually  to  secondary  buds ;  these  become 
sexual,  and  their  fertilised  ova  give  rise  to  the  original  "nurse" 
forms.  There  are  therefore  several  asexual  generations  between 
the  sexual,  and  our  formula  must  run, — 


204  THE    EVOLUTION    OF    SEX. 

§  3.  Alternation  bctiveen  Sexual  and  Degenerate  Sexual  Reproduction, — 
The  cases  we  have  just  noticed  are  both  easier  to  state  and  easier  to  explain 
than  others  which  are  sometimes  also  included  under  the  vague  title  of 
"  alternation  of  generations."  The  above  alternations  were  between  sexual 
and  asexual  reproduction ;  these  must  be  distinguished,  vague  as  the 
boundary  must  be,  from  alternation  between  the  ordinary  sexual  process 
and  a  degenerate  form  of  the  same. 

The  adventurous  history  of  some  of  the  flukes  {Trcuiatoda)  may  be  taken 
as  a  first  illustration.  The  common  liver-fluke  {Disfo/unin  or  Fasciola 
hepat ic a) -wYiich.  causes  the  disastrous  *'rot''  in  sheep  has  a  life  of  vicis- 
situdes. The  fertilised  ovum  gives  rise  to  an  embryo,  which  passes  from 
the  sheep  which  its  sexual  parent  infested  to  the  water  by  the  field  side. 
There  it  leads  for  a  while  an  active  life,  knocking  against  many  things,  but 
finally  attaching  itself  to  a  minute  water-snail.  Into  this  it  bores,  losing 
its  covering  of  active  cilia  with  change  of  habit,  and  becoming  much 
altered  into  a  passive  vegetative  form  known  as  a  sporocyst.  Now  this 
sporocyst  sometimes  divides  ;  and  if  this  were  all,  and  the  results  grew  up 
into  liver-flukes,  we  should  have  the  old  formula  and  less  loss  of  sheep. 
But  direct  development  never  occurs,  and  we  may  leave  the  casual  division 
at  present  out  of  account.  Certain  cells  within  the  sporocyst  form  germs, 
and  these  serve  in  the  place  of  genuine  ova.  They  produce  within  the  body 
of  the  sporocyst  another  brood  of  what  are  called  Rediiv.  There  may  be 
several  generations  of  them,  and  the  final  result  is  a  brood  of  minute  tailed 
organisms  [Ceirariic),  which  leave  the  water-snails,  leave  the  water  even, 
creep  up  grass  stems,  and  encyst  themselves.  There  most  wait  for  death, 
a  few  for  the  attainment  of  adult  sexual  life  if  they  chance  to  be  eaten  by  a 
sheep.     The  somewhat  complex  story  may  be  written  in  lines  : — 

The  fertilised  ovum  gives  rise  to  an  aquatic  embryo  (i). 

This  enters  a  water-snail,  and  becomes  a  ^''  sporocyst  J''' 

(The  sporocyst  may  divide.) 

Within  the  sporocyst,  cells  develop  into  ^^  Rediiv'^  (2). 

There  may  be  several  generations  of  redi.x  (3,  4). 

The  last  generation  {Cercaria)  may  become  adult  sexual  liver-flukes  (5). 

This  cannot  be  accurately  ranked  as  parallel  to  what  occurs  among 
the  above-mentioned  tunicates,  for  the  redire  arise  from  precocious  repro- 
ductive cells.  These  cannot  be  called  ova,  and  there  is  no  fertilisation,  but 
yet  the  process  is  not  one  of  division,  or  of  budding.  It  is  a  degenerate 
process  of  parthenogenetic  reproduction  in  early  life.  The  facts  may  be 
again  summed  up  in  a  formula,  which  does  not  take  account  of  the  occa- 
sional division  of  the  "  sporocyst." 


A,  asexual  larvae ;  S,  sexual  fluke  ;  the  upper  circles  represent 
the  special  germ  cells  ;  fertilised  ova  at  the  base. 

The  germ-cells,  which  behave  like  ova,  and  yet  do  not  rise  to  that  level, 
appear  sometimes  in  a  central  mass  within  the  asexual  individual,  some- 


ALTERNATION  OF  GENERATIONS.  205 

times  simply  in  the  epithelium  lining  the  body  walls.  There  may  be  a 
long  series  of  generations  producing  and  produced  in  this  way,  and  these 
are  often  unlike  one  another.  Fluke,  embryo,  sporocyst,  redia,  and  cer- 
caria,  are  all  markedly  different  in  structure,  though  embryo  changes  into 
sporocyst,  and  cercaria  into  fluke. 

This  alternation  between  sexual  reproduction  with  the  usual  fertilisation, 
and  reproduction  by  means  of  special  cells  which  yet  require  no  fertilisa- 
tion, prevails  in  many  plants,  e.^.^  ferns  and  mosses.  From  a  fertilised 
egg-cell  the  ordinary  fern-plant,  with  which  everyone  is  familiar,  develops. 
But  this  is  quite  asexual,  if  we  mean  by  that  that  it  is  neither  male  nor 
female,  and  that  it  produces  neither  male  nor  female  elements.  At  the 
same  time  it  produces  special  reproductive  cells, — not  egg-cells  exactly,  any 
more  than  those  within  the  sporocyst  were,  but  yet  able  to  develop  of 
themselves  into  a  new  organism.  This  is  not  another  fern-plant,  however, 
but  an  inconspicuous  green  organism,  much  less  vegetative,  and  sexual. 
The  so-called  "  spore"  formed  on  the  leaves  of  the  sexless  fern-plant  falls 
to  the  ground,  develops  a  "  prothallus,"  which  bears  male  or  female  organs, 
or  both.  An  egg-cell  is  fertilised  by  a  male  element,  and  the  conspicuous 
vegetative  fern-plant  once  more  arises.  The  formula  is  therefore  as 
follows  : — 


Where  A  =  sexless  vegetative  fern-plant  ; 

sp.  =  the  parthenogenetic  special  reproductive  cell  or  spore  ; 
S  =  the  sexual  inconspicuous  "  prothallus,"  with  male  and  female  organs. 

Now  take  the  history  of  a  moss.  Unlike  the  fern,  the  more  con- 
spicuous "moss-plant"  is  sexual.  It  bears  male  and  female  organs,  and 
an  egg-cell  is  fertilised  by  a  male  element.  The  fertilised  egg-cell,  how- 
ever, does  not  lose  its  hold  of  the  mother  plant,  but  grows  like  an  encum- 
bering parasite  upon  it.  Obviously,  then,  it  does  not  give  rise  to  another 
"  moss-plant."  The  result  of  the  fertilised  egg-cell  is  a  tiny  sexless  stalk, 
which  bears  on  its  apex  the  special  reproductive  cells  or  spores  with  which 
we  are  now  familiar.  In  other  words,  the  fertilised  egg-cell  develops  into 
a  parasitic  spore-bearing  generation.  The  "spores"  fall  into  the  ground, 
as  they  did  in  the  fern,  and  there  grow  into  a  usually  thread-like  structure, 
from  which  the  sexual  moss-plants  are  budded  off.  If  we  do  not  emphasise 
the  transitional  thread-like  stage, — the  protonema  as  it  is  called, — the 
formula  is  as  follows  (see  also  fig.  p.  201) : — 


Where  A=  inconspicuous  sexless  parasitic  generation  upon  the  "  moss-plant." 

sp.  =i  the  special  parthenogenetic  reproductive  cell  or  spore  produced  by  A. 
S  =  the    conspicuous    sexual     "moss-plant,"     budded     from     the     threads 
developed  from  the  spore. 


2o6 


THE    EVOLUTION    OF    SEX. 


If  we  do  emphasise  the  "protonema"  stage  (/),  and  regard  the  moss- 
plants  as  asexually  budded  from  it,  the  formula  runs  :  — 


r^^^-L 


In  the  fern,  the  vegetative  sexless  generation  was  the  more  conspicuous  ; 
in  mosses,  the  sexual  generation.  In  a  way  this  recalls  the  contrast 
between  the  life-history  of  many  a  zoophyte,  and  that  of  the  common 
jelly-fish  aurelia.  The  asexual  hydroid  colony  is  more  conspicuous  than 
the  usually  small  swimming-bell,  but  the  sexual  jelly-fish  is  much  more 
conspicuous  than  the  minute  asexual  "hydra-tuba."  The  common  com- 
parison between  medusoid  and  hydroid  on  the  one  hand,  and  prothallus 
and  fern-plant  on  the  other,  is  rather  misleading,  simply  because  the 
hydroid  merely  Inids  off  the  medusoid,  while  the  fern-plant  produces  the 
prothallus  from  a  special  reproductive  cell  or  spore.  In  some  ferns  and 
mosses,  however,  a  more  exact  parallel  is  occasionally  exhibited.  The 
production  of  "  spores  "  may  be  suppressed,  and  from  the  place  where  they 
should  be  formed  a  (sexual)  fern-prothallus  or  a  new  (sexual)  moss-plant  is 
vegetatively  developed,  just  as  medusoid  from  hydroid.  This  exceptional 
occurrence  is  technically  called  apospory.  The  very  opposite  of  this  also 
occurs,  the  suppression  not  of  the  spore  bearing,  but  of  the  sexual  genera- 
tions. The  fern-plant  then  arises  vegetatively  from  the  prothallus  ;  and 
this  would  be  paralleled  if  we  supposed  the  sporocyst  of  the  fluke  to  Imd 
off  redice  (as  it  sometimes  does),  and  these  to  continue  the  species  without 
ever  becoming  really  sexual,  solely  by  means  of  the  special  cells  above 
described. 

§  4.  Cofubinaiion  of  both  tliese  Alternations. — The  asexual  hydroid  buds 
off  a  medusoid,  the  fertilised  ovum  of  which  develops  into  a  hydroid. 
Here  there  is  simple  alternation  between  sexual  and  asexual  reproduction. 


A  sexless  fcrn-[)lant  forms  sjiecial  reproductive  cells  (sjiorcs),  which 
develop  parthenogenelically  into  a  sexual  prothallus,  from  the  fertilised 
egg-cell  of  which  the  fern-plant  arises. 


ALTERNATION  OF  GENERATIONS.  207 

The  difference  between  these  two  alternations  has  been  as  often  pointed 
out  as  it  has  been  ignored.  The  former  is  called  true  alternation  of 
generations  (or  metagenesis)  ;  the  latter  is  called  by  zoologists,  in  reference 
to  flukes  for  instance,  heterogamy.  Comparisons  between  the  alternations 
in  plants  and  animals  have  seldom  recognised  the  distinction. 

Let  it  be  recognised,  however,  and  we  can  readily  proceed  to  more 
complicated  cases  where  the  two  are  combined.  Returning  to  the  liver- 
fluke  and  others  like  it,  we  find  that  the  sporocyst  sometimes  multiplies  in 
a  genuinely  asexual  fashion — without  the  intervention  of  precocious  ova, 
special  reproductive  cells,  germs,  or  spores,  call  them  what  you  will — 
by  direct  division  or  budding.  For  such  cases  the  formula  must  l)e 
modified  as  follows  : — 


The  complication  is  not  serious.  It  is  simply  that,  before  the  multipli- 
cation by  special  cells  sets  in,  there  may  be  more  than  one  (A',  A")  entirely 
asexual  (and  not  merely  sexless)  generation. 

§  5.  Altei-nation  of  Jtivettile  Parthenogenetic  Reproduction  unth  the 
Adult  Sexual  Process — We  have  already  noted  the  curious  precocity  of  some 
midge  larvoe,  which  reproduce  while  still  young.  Cells  within  the  body, 
apparently  precocious  ova,  develop  parthenogenetically  into  larvae,  which 
prey  upon  the  mother  larva,  eventually  kill  her  and  leave  her,  only  themselves 
to  become  in  turn  similar  victims  of  precocity.  This  may  continue  for  a 
series  of  generations,  with  continuous  decrease  in  the  size  of  the  reproduc- 
tive cells,  till  finally  true  sexuality  and  adult  life  is  attained.  The  repro- 
ductive cells  here  are  rather  more  differentiated  than  those  in  the  young 
flukes,  but  the  close  parallelism  is  indubitable.  Except  that  there  is  for  a 
while  no  fertilisation,  the  process  can  hardly  be  called  asexual.  The 
formula  may  be  expressed  in  a  gentle  curve  : — 


Where  the  starting  point  is  as  before  a  fertilised  ovum  ; 
L  —  prematurely  reproductive  larva  ; 
ps  =  precocious  parthenogenetic  "  pseudova  "  ; 
S  =  adult  sexual  male  or  female  organism. 

Somewhat  different  is  the  curious  case  of  Gyrodactyhis,  a  trematode 
parasitic  on  fresh-water  fishes,  where  three  generations  are  found  enclosed, 
one  within  the  other,  in  a  fashion  which  recalls  the  fancies  of  the  preforma- 
tionists.  In  this  case,  however,  it  seems  likely  that  internal  fertilisation 
really  occurs. 

§  6.  Alternaiioii  of  Parthenogenesis  and  Ordinary  Sexual  Reprodiution. 
— In  our  gradual  ascent,  we  now  reach  the  frequent  alternation  of  partheno- 
genesis and  ordinary  sexual  reproduction.  The  special  cells  which  develop 
without  fertilisation  are  now  genuine  parthenogenetic  ova,  and  the  organisms 
which  produce  them  are  adults,   not  juveniles.      The  formulae  will  differ 


2o8  THE    EVOLUTION    OF    SEX. 

mainly  in   the  number  of  generations  through  which  the  parthenogenesis 
may  be  ccMitinued. 


Where  the  starting-point  is  a  fertilised  ovum. 

P  =  parthenogenetic   female,  producing   a  parthenogenetic 

ovum,  from  which  arise  other  parthenogenetic  forms, 

or  eventually 
S=^male  and  female. 

§  7.  Alternation  of  Different  Scxnal  Generations. — The  rhythm  may 
be  followed  in  yet  a  higher  scale.  In  a  very  few  cases  there  is  an  alter- 
nation between  two  different  sexual  generations.  Thus  one  of  the  thread- 
worms {Leptodera  appendiciilata)  found  in  the  snail  gives  rise,  by  the  or- 
dinary  sexual  process,  to  a  different  form,  which  leads  a  free  life,  and 
subsequently  gives  origin  to  the  parasite.  In  both  generations  the  sexes 
are  distinct.  More  remarkable  still  is  the  history  of  another  nematode 
{Angiostoniiim  nigrovenosiim)^  found  in  the  lung  of  the  frog.  It  is  physio- 
logically hermaphrodite,  though  its  organ  is  ovary-like ;  its  eggs  are 
fertilised  by  its  own  sperms,  which  mature  tirst ;  the  progeny  become  sexual 
— males  and  females — in  the  earth,  and  their  offspring  return  to  the  frog, 
where  they  become  hermaphrodites.  Another  example  of  alternation  of 
sexual  generations  is  found  in  one  of  the  threadworms  which  occur  in  man 
{ Rhabdonenia  strongyloides). 

§  8.  Occ7irrenee  of  these  Alternations  in  Animals. — From  sponges  to 
tunicates  such  alternations  occur.  Beyond  the  latter,  unless  we  wish  to  be 
very  subtle,  they  cease.  It  is  necessary  to  be  clear  about  the  fact  that 
asexual  and  sexual  reproduction  may  occur  together  in  the  same  form. 
The  common  hydra  gives  off  buds  in  an  entirely  asexual  way,  but  it  is  also 
a  sexual  animal,  with  male  and  female  organs.  There  may  be  periods  of 
vegetative  growth  and  climacterics  of  sexuality  in  the  same  organism,  with- 
out any  alternation  of  generations. 

It  is  possible  that  the  term  alternation  of  generations  may  be  applied 
to  some  of  the  phenomena  observed  in  the  Protozoa.  Thus  Brandt  main- 
tains that  all  the  colonial  radiolarians,  known  as  Sphcerozoa,  form  on  the 
one  hand  isospores,  which  are  all  equal  and  apparently  parthenogenetic, 
and  on  the  other  hand  anisospores,  which  are  large  and  small, — in  fact, 
sexually  dimorphic.  He  believes — though  the  fact  cannot  be  called 
demonstrated — that  two  unequal  anisospores  unite  to  form  a  double  cell,  a 
fertilised  unit,  which  will  produce  isospores  again,  and  these  the  normal 
colony.  The  generation  of  these  sphairozoa  is  further  complicated  {a)  by 
division  of  the  colonies,  {l>)  by  division  of  the  individuals  of  young  vegetative 
colonies,  and  {c)  by  the  formation  of  special  "  extra-capsular  "  reproductive 
bodies  in  young  colonies. 

The  history  of  the  common  fresh-water  sponge  [Spongilla),  as  told  by 


ALTERNATION    OF    GENERATIONS.  209 

Marshall,  is  one  of  many  vicissitudes.  In  autumn  the  sponge  begins  to 
suffer  from  the  cold  and  scarcity  of  food.  It  dies  away  ;  but  some  of  the 
units  save  themselves,  and,  in  a  sense,  the  parent,  by  forming  the 
"  gemmules"  we  have  already  noticed.  These  winter  in  a  quiescent  state 
within  the  parental  corpse,  but  in  spring  they  get  out  of  the  debris,  and 
start  male  or  female  sponges.  The  males  are  short-lived,  but  their  male 
elements  fertilise  the  ova  of  the  females.  The  fertilised  ovum  develops 
into  a  ciliated  embryo,  and  this  into  an  asexual  sponge,  which  produces  the 
gemmules. 


The  starting-point  a  fertilised  ovum,  which  develops  into 
A  =  asexual  sponge,  which  forms  only 
G—  gemmules,  which  develop  into 
S  -^  male  and  female  sponges. 

Besides  the  hydroid  and  medusoid,  the  hydra-tuba  and  jelly-fish  alterna- 
tions, which  we  have  already  noticed,  there  are  many  complications  of 
degree  among  C(ielenterates.  The  medusoid  stage  degenerates  by  subtle 
gradations,  ceasing  to  be  free,  and  eventually  becoming  what,  if  its 
history  were  not  known,  would  be  called  an  organ  rather  than  a  "  person  " 
of  the  colony.  Furthermore,  it  may  itself  take  to  budding,  and  continue 
the  asexual  habit  of  the  hydroid  from  which  it  springs.  Outside  the 
Hydrozoa,  genuine  alternation  of  generations  does  not  occur,  unless  that 
described  by  Semper  for  P'ungia  corals  be  accepted  as  such. 

A  very  interesting  alternation  has  been  recently  described  by  W.  K. 
Brooks  in  a  remarkable  medusa  {Epenthesis  macradyi).  On  the  reproduc- 
tive organs  of  this  swim-bell  there  grow,  like  parasites,  what  are  exactly 
comparable  to  the  reproductive  buds  (blastostyles)  of  a  hydroid,  and  these 
form  medusoids  by  budding.  The  result  is  a  compound  colony,  which 
approaches  the  Siphonophora.  The  process  recalls  and  surpasses  the 
apogamy  of  a  few  ferns. 

Among  worm- types,  the  strict  alternation  of  generations  in  some  of  the 
marine  chstopods  (syllids),  the  more  complicated  phenomena  of  so  many 
trematodes,  the  sexual  rhythms  of  that  peculiar  threadworm  Aiigiostomtim^ 
have  been  already  discussed.  It  is  necessary,  however,  to  state  the  case  for 
tapeworms,  which  are  usually  included  among  the  examples  of  alternation 
of  generations.  The  usual  view  is,  that  the  embryo  of  a  tapeworm  develops 
into  an  asexual  bladder- worm,  which  asexually  buds  off  a  "  head,"  or  more 
than  one.  Such  a  "  head,"  passing  to  another  host,  buds  off  asexually 
the  chain  of  reproductive  joints  or  sexual  individuals  which  constitute  a 
tapeworm.  Asexual  bladder-worm,  asexual  "  head,"  and  sexual  joints, 
form  the  series.  That  there  is  a  genuine  alternation  of  generation  is 
believed  by  some  authorities,  but  there  are  emphatic  difficulties  against 
this  supposition,  except  in  the  occasional  occurrence  of  a  bladder-worm 
with  several  "heads,"  each  of  which  may  develop  into  a  tapeworm.  The 
case  is  well  stated  by  Hatchett  Jackson  in  his  monumental  edition  of 
Rolleston's  *'  Forms  of  Animal  Life,"  and  we  accept  his  verdict  that  there 
is  really  one  individual  throughout,  except  when  asexual  multiplication  of 

O 


210  THE    EVOLUTION    OF    SEX. 

heads  occurs.  The  tapeworm,  on  this  view,  is  an  adult  sexual  bladder- 
worm,  and  the  joints  are  only  highly  individualised  segments. 

Of  the  parthenogenetic  cycles  in  crustaceans  and  insects,  the  juvenile 
reproduction  of  some  of  the  latter,  and  the  true  alternation  of  generations 
in  some  tunicates,  enough  has  already  been  said. 

Von  Jhering  is  responsible  for  starting  the  paradox,  that  in  higher 
animals  a  mother  may  Ijring  forth  her  grandchildren.  He  refers  to  the 
case  of  the  hya^na-like  carnivore  Praopus,  where  a  single  ovum  gives  rise  to 
eight  embryos,  which  are  thus  in  a  pedantic  sense  grandchildren  !  The 
frequent  occurrence  of  twins  in  all  groups,  the  remarkable  case  of  an  earth- 
worm {Liiiiibricus  trapczoidcs)  in  which  a  doul)le  embryo  is  constant,  and 
the  morphological  resemblance  of  polar  globules  to  abortive  germs,  led  Von 
Jhering  to  maintain  that  the  origin  of  multiple  embryos  from  a  single 
ovum  is  the  primitive  and  normal  condition,  and  that  the  development  of 
only  one  is  secondary  and  adaptive.  The  data  are  hardly  sufficient  for  such 
a  striking  conclusion. 

§  9.  Occurrence  of  Alternations  in  Plants. — In  the  lower 
plants,  alga^  and  fungi,  an  alternation  between  spore-producing 
and  truly  sexual  generations  is  frequent.  In  mosses  and  ferns  it 
is  almost  constant,  and  yet  more  marked.  Occasionally  either 
spore-formation  or  sex-cell  formation  may  be  suppressed,  and 
the  life-history  thus  simplified.  In  a  few  of  the  higher  plants 
both  are  exceptionally  sup]:)ressed,  and  we  have  thus  a  reversion 
to  a  purely  vegetative  process,  just  as  if  a  hydra  went  on  giving 
off  daughter-buds  without  ever  becoming  sexual.  In  the 
flowering  })lants,  what  corresponds  to  the  sexual  generation  of  a 
fern  is  much  reduced ;  it  has  come  to  remain  continuous  with 
the  vegetative  asexual  generation,  on  which  it  has  reacted 
in  subtle  physiological  influence.  Just  as  in  the  higher  animals, 
alternation  of  generations  finds  at  most  only  a  rudimentary 
expression. 

S^  10.  Heredity  in  Alter7iating  Generations. — The  problem 
of  the  relative  constancy  of  inheritance  is  now  in  part  solved  by 
the  theory  of  germinal  continuity.  The  ovum  which  develops 
into  an  offspring  is  virtually  continuous,  either  in  itself  or 
through  its  nucleus,  with  the  ovum  which  gave  rise  to  the 
parent.  A  chain  of  ovum-like  cells  is  only  demonstrable  in  a 
few  cases;  but  Weismann  overcomes  this  difficulty,  by  supposing 
that  what  really  keeps  up  the  protoplasmic  tradition  or  con- 
tinuity between  the  parental  ovum  and  the  next  generation,  is  a 
si)ecific  and  stable  portion  of  the  nucleus, — the  "  germ-plasma." 
When  a  medusoid  goes  off  from  a  hydroid,  it  carries  with  it  a 
legacy  of  this  germ-plasma,  continuous  with  that  which  gave 
rise  to  the  hydroid.  This  legacy  forms  the  reproductive 
elements  of  the  medusoid,  which  in  turn  give  rise  to  hydroids. 


ALTERNATION    OF    GENERATIONS. 


211 


The  medusoid  itself  is  a  modified  asexual  growth,  into  which 
some  of  the  germ-plasma  of  the  hydroid  has  migrated ;  it  is 
literally  only  the  bearer  of  the  hydroid  germ-plasma.  Weis- 
mann's  classic   researches   on  hydroids  have    shown  that  the 


I.  The  hermaphrodite  fern  prothaUus  contrasted  with  (2  a)  the  male  and  (2  d) 
the  female  thallus  of  li\er\vort,  and  (3  a  and  /')  male  and  female  prothaUus  of 
horsetail.  Above  are  the  corresponding  reductions  of  the  sexual  prothallia 
in  (4)  Salvinia,  (5)  Isoetes,  (6)  Cycad  and  Conifer,  and  (7)  Phanerogam. 

reproductive  cells,  which  by  hypothesis  bear  the  germ-plasma, 
often  arise  far  down  in  the  hydroid  body,  and  actually  migrate 
to  their  final  seat  in  the  bearer.  Where  the  alternation  is  not 
between  sexual  and  asexual,  but  between  the  ordinary  sexual 


2  12  THE    EVOLUTION    OF    SEX. 

process  and  multiplication  Ijy  si)ecial  })arthenogenetic  cells,  as 
is  the  case  in  many  flukes,  we  are  in  the  same  way  bound  to 
sup[)ose  that  the  cells  within  a  sporocyst  which  give  rise  to 
redii^  are,  like  ova,  charged  with  this  reproductive  germ-plasma. 
It  is  very  interesting  to  notice  that,  as  far  back  as  1849, 
Owen  had  a  distinct  prevision,  not  only  of  the  distinction 
between  body-forming  cells  and  reproductive-cells,  of  which 
so  much  is  now  made,  but  of  the  essential  idea  of  the  "  germ- 
plasma."  Speaking  of  the  recurrence  of  a  parental  form  after 
numerous  interpolated  generations,  he  says,  "the  essential  con- 
dition is  the  retention  of  ceriain  of  the  progeny  of  the  primary 
impregnated  germ-cell,  or,  in  other  words,  of  the  germ-mass 
unchanged  in  tiie  body  of  the  first  individual  developed  from 
that  germ-mass,  with  so  much  of  the  spermatic  force  inherited 
by  the  retained  germ-cells  from  the  parent-cell  or  germ-vesicle 
as  suffices  to  set  on  foot  and  maintain  the  same  series  of 
formative  actions  as  those  which  constituted  the  individual 
containing  them."  In  this  somewhat  over-weighted  sentence, 
if  we  read  "  germ-]:)lasma  "  instead  of  "  spermatic  force,"  we 
have  a  close  approximation  to  the  modern  conception  of 
Weismann.  So  again,  he  says,  "  an  impregnated  germ-cell 
imparts  its  spermatic  power  to  its  cell-offspring;  but  when 
these  perish,  or  when  the  power  is  exhausted  by  a  long  descent, 
it  must  be  renewed  by  fresh  impregnation.  But  nature  is 
economical,  and  so  long  as  sufficient  power  is  retained  l)y  the 
progeny  of  the  primary  impregnated  vesicle  (the  essential  part 
of  an  ovum),  individuals  are  developed  from  that  progeny 
without  the  recurrence  of  the  impregnating  act." 

§  II.  Hints  as  to  the  Rationale  of  Alternation. — We  shall 
have  to  take  a  fresh  view^  of  alternation  of  generations  after  the 
general  theory  of  growth  and  reproduction  has  been  discussed ; 
meanwhile,  however,  the  physiological  aspect  of  the  facts  may 
be  simply  indicated.  A  fixed  hydroid  contrasted  with  a 
swimming-bell  or  medusoid,  a  sessile  hydra-tuba  contrasted 
with  an  actively  locomotor  jelly-fish,  illustrate  not  a  peculiar 
antithesis,  but  a  most  general  and  fundamental  rhythm  of 
organic  life, — that  between  nutrition  and  reproduction.  The 
hydroid  has  a  relatively  passive  habit  and  a  copious  nutrition ; 
it  is  })reponderatingly  vegetative  and  asexual.  The  reverse 
habit,  the  physiological  rebound,  finds  exi)ression  in  the 
medusoid.  In  the  same  way,  though  the  alternation  is  less 
strictly  between  asexual  and  sexual,  the  contrast  between  leafy 


ALTERNATION  OF  GENERATIONS. 


213 


spore-bearing  fern-plant  and  inconspicuous  sexual  prothallus 
is  again  fundamentally  parallel.  The  notation  adopted  must 
have  already  suggested  our  fundamental  diagram,  the  different 
forms  of  which  may  be  separated  out  or  superposed  : — 


SUM    OF    FUNCTIONS. 


Nutrition. 


Reproduction. 


Anabolism.     Katabolism.     Female. 


Male. 


Although  it  has  just  been  shown  that  the  process  of  alter- 
nation demands  a  much  more  thorough  analysis  and  discrim- 
ination of  the  different  cases  than  has  hitherto  been  customary, 
and  this  on  the  physiological  as  well  as  merely  on  the  morpho- 
logical side,  the  general  aspect  of  the  process,  in  which  an 
asexual  form  alternates  with  one  or  more  dimorphic  sexual 
generations,  makes  it  evident  that  we  have  here  to  do  in  two 
generations  with  what  is  often  so  ol)vious  in  one, — the  familiar 
antithesis  between  nutrition  and  reproduction.  A  consideration 
of  the  physiological  distinctions  between  the  asexual  and  sexual 
generations,  shows  that  the  former  is  the  expression  of  favour- 
able nutritive  conditions  resulting  in  vegetative  growth,  or  at 
most  in  asexual  multiplication,  while  the  latter  is  conditioned 
by  less  propitious  circumstances.  Just  as  a  well-nourished 
plant  may  continue  propagating  itself  by  shoots  and  runners, 
and  just  as  an  aphis  in  artificial  sunnner  may  for  years  repro- 
duce parthenogenetically,  so  a  hydroid  with  abundant  food  and 
otherwise  favourable  environment  may  be  retained  for  a  pro- 
longed period  vegetative  and  asexual,  while  dearth  of  food  and 
otherwise  altered  conditions  evoke  the  appearance  of  the  sexual 
generation.  The  contrast  between  the  deeply-rooted  well- 
expanded  fern-plant  and  the  weakly-rooted  slightly-exposed 
prothallus,  is  obviously  that  between  an  organism  in  conditions 
favourable  to  the  continuance  and  preponderance  of  anabolic 


2  14  THE    EVOLUTION    OF    SEX. 

processes,  and  an  organism  in  an  environment  where  katabolism 
is,  at  an  early  stage,  likely  to  gain  the  ascendant.  The  former 
is  thus  naturally  asexual,  the  latter  sexual.  A  survey,  in  fact, 
of  the  conditions  and  characteristics  of  the  two  sets  of  forms, 
inevitably  leads  us  to  regard  the  asexual  generation  as  the  ex- 
pression of  predominant  anabolism,  and  the  sexual  as  equally 
emphatically  katabolic.  Alternation  of  generations  is,  in  fine, 
a  rhythm  between  'a  relatively  anabolic  and  katabolic  prepon- 
derance. 

§  12.  Ofight  of  Alternation  of  Generations. — Even  in  an  individual 
plant  or  animal  there  are  vegetative  and  reproductive  periods  ;  alternation 
of  generations  involves  the  separation  of  these  to  different  individuals,  by  the 
interpolation  of  more  or  less  asexual  reproduction.  In  most  hydroids,  the 
asexual  vegetative  tendency  preponderates  ;  in  most  medusoids,  the  sexual 
reproductive  dominates.  But  the  origin  in  each  particular  case  is  involved 
in  the  pedigree  of  the  organism.  Thus  Hojckel  distinguishes  a  progressive 
from  a  retrogressive  origin  ;  in  the  former,  the  organisms  are  in  transition 
from  preponderant  asexual  to  sexual  reproduction ;  in  the  latter,  the  organisms 
are  returning  or  degenerating  from  dominant  sexuality  to  an  asexual  pro- 
cess. It  is  safe  to  say  that  the  latter  is  more  frequently  the  right  inter- 
pretation of  the  facts.  So  far  as  reproduction  is  concerned,  one  of  those 
vi\Qd\\'io\(\?,{7rach)n/iediiS(v)\^\i\c\\  have  no  corresponding  hydroid  parent, 
or  a  jelly-fish  like  Pelagia  which  has  no  fixed  asexual  hydra-tuba  stage,  is 
nearer  the  ancestral  habit  than  those  members  of  both  divisions  which 
exhibit  alternation  of  generations.  Where  we  have  alternating  series  of 
similar  forms  with  different  degrees  of  sexuality,  e.g.^  the  rhythm  between 
parthenogenesis  and  true  sexual  reproduction  in  aphides,  Weismann  once 
interpreted  the  facts  as  associated  with  the  periodic  action  of  external  influ- 
ences ("  Studies  in  the  Theory  of  Descent,"  chap.  v.).  But  in  contrast  to 
such  cases  he  distinguished,  (rr)  an  origin  from  metamorphosis,  where  one 
stage  in  the  life-history  becomes  precociously  reproductive,  e.g.,  in  the 
midge  Cecuio7nyia ;  (/;)  the  case  of  the  Hydromedusae,  where  sexuality  is 
postponed  in  early  life,  and  asexual  reproduction  dominates  ;  and  (r)  an 
origin  from  division  of  labour  within  a  colony.  Without  entering  upon  a 
discussion  of  each  case  in  relation  to  its  history  and  environment,  it  is  not 
possible  to  do  more  than  reassert  that  in  many  different  degrees  the  con- 
tinuous alternation  between  growth  and  multiplication,  nutrition  and  repro- 
duction, asexuality  and  sexuality,  anabolism  and  katabolism,  may  express 
itself  in  the  life-history  of  the  organism. 

Postscript. — From  Mr  R.  J.  Harvey  Gibson's  valuable  paper  on  "  The 
Terminology  of  the  Reproductive  Organs  of  Plants"  (Proc.  Liverpool  Biol. 
See,  Vols.  III.  and  IV.),  we  take  the  following  scheme  : — 

A.  Asexual  stage  or  sporophyte,  produces  spores  in  sporangia  [ovospo?-- 
angia  and  sperniosporangia  in  higher  Cryptogams  and  Phanerogams). 

B.  Sexual  stage  ox  ga/iiophyte  {oop/iyte  and  spernwphyte  where  the  thallus 
is  unisexual),  produces  ova  and  sperms  in  ovaries  and  speriuat-ies ;  the  pro- 
duct of  union  of  ovum  and  sperm  being  an  oosperm. 


ALTERNATION    OF    GENERATIONS.  215 


SUMMARY. 

1.  The  fact  that  successive  generations  may  be  markedly  different  was 
observed  by  the  poet  Chamisso,  and  first  made  precise  by  the  zoologist 
Steenstrup. 

2.  A  fixed  asexual  hydroid  buds  off  and  liberates  locomotor  sexual 
swimming-bells,  whose  fertilised  ova  give  rise  again  to  hydroids.  Asexual 
and  sexual  generations  alternate. 

3.  The  offspring  of  the  liver-fluke  forms  from  certain  cells  in  its  body 
a  numerous  progeny  ;  these  repeat  the  same  process  several  times  ;  the  last 
generation  grow  into  the  sexual  liver-flukes.  Reproduction  by  special 
cells  like  precocious  undifferentiated  ova,  alternates  with  reproduction  by 
ordinary  fertilised  ova.  So  too  the  vegetative  sexless  "  fern-plant "  gives 
rise  to  special  cells  like  parthenogenetic  egg-cells,  which  develop  into  an 
inconspicuous  sexual  prothallus.  From  the  fertilised  egg-cell  of  the  latter 
the  "  fern-plant  "  arises. 

4.  These  two  different  kinds  of  alternations  (§  2  and  §  3)  may  be  com- 
bined in  a  more  complicated  manner. 

5.  In  some  flies  precocious  parthenogenetic  reproduction  alternates  with 
the  normal  sexual  reproduction  of  the  adults. 

6.  In  many  insects  and  crustaceans,  parthenogenetic  reproduction  alter- 
nates with  the  normal  sexual  process.  There  may  be  one  or  many  inter- 
vening parthenogenetic  generations. 

7.  A  hermaphrodite  threadworm  parasitic  in  the  frog  fertilises  its  own 
eggs,  which  develop  into  free-living  males  and  females,  from  the  fertilised 
ova  of  which  the  hermaphrodite  parasites  again  arise.  Here  there  is  an 
alternation  of  sexual  generations. 

8.  In  animals  these  alternations  occur  from  sponges  up  to  tunicates. 

9.  In  plants  they  occur  in  a\gve  and  fungi,  are  almost  constant  in  ferns 
and  mosses,  but  are  inconspicuous  in  higher  plants. 

10.  The  problem  of  heredity  is  somewhat  complicated  by  such  alter- 
nations. 

11.  Alternation  of  generations  is  but  a  rhythm  between  a  relatively 
anabolic  and  katabolic  preponderance. 

12.  The  origin  has  varied  considerably  in  different  cases. 


LITERATURE. 

See  the  general  works  already  cited  ;  also,  Steenstrup  "  On  the  Alternation 
of  Generations,"  transl.  Ray  Soc,  1845  ;  Owen's  "Parthenogenesis," 
&c.,  1849;  Hceckel's  "  Generelle  Morphologic,"  1866;  Weismann, 
A.,  Die  Entstehung  der  Sexualzellen  bei  den  Ilydromedusen,  Jena, 
1883  ;  and  Papers  on  Heredity,  Translation,  Oxford,  1889  ;  Vines' 
article  "Reproduction — Vegetable,"  Ency.  Brit.;  and  the  ordinary 
Text-books  of  Zoology  and  Botany. 


BOOK    IV. 


THEORY   OF    REPRODUCTION. 


CHAPTER  XVI. 

Growth  and  Reproduction. 

§  I.  Facts  of  Growth. — In  a  well-known  aphorism  Linnaeus 
noted  that  living  organisms  were  not  alone  in  their  power  of 
growth.  Crystals  become  centres  for  other  crystals,  till  a  large 
mass  results  ;  and  the  product,  as  every  case  of  minerals  shows,  is 
often  both  orderly  and  complex.  But  it  can  hardly  be  said  that 
an  inorganic  body  has  any  control  over  or  credit  in  its  growth, 
nor  does  the  latter  follow  as  the  almost  necessary  consequence 
of  previous  waste  or  liberation  of  energy.  It  is  one  of  the  oldest 
generalisations,  that  the  growth  of  organisms  has  a  peculiar 
method  of  its  own,  that  of  intussusception  as  distinguished  from 
mere  accretion.  The  new  particles  which  are  taken  in,  more  than 
replacing  previous  expenditure,  are  not  deposited  upon  the  sur- 
face of  already  established  material,  as  is  the  case  with  a  crystal, 
but  are  intercalated  in  the  interstices  of  previous  particles.  It 
is  of  course  unnecessary  to  enter  here  upon  the  long-continued 
controversy,  whether  such  structures  as  the  cell-wall  and  starch- 
grains  of  plants  grow  thicker  or  larger  by  accretion  in  crystal- 
like fashion,  or  by  intercalation  which  is  supposed  to  be  charac- 
teristically organic.  It  is  worth  noticing,  however,  as  Biitschli 
points  out,  that  if  the  living  matter  has  the  foim  of  an  intricate 
network,  the  fresh  material  of  replacement  or  growth  may  be 
added  to  the  surfaces  of  the  threads  which  make  the  web.  Thus 
what  is  roughly  called  intercalation  may  be  more  literally  an 
internal  accretion. 

Hunger  is  a  dominant  characteristic  of  living  matter.  When 
a  unit  mass  or  cell  has  been  giving  off  energy  in  doing  any  kind 
of  work,  its  substance  is  chemically  impaired, — less  capable  of 
doing  further  work  until  new  energy  has  been  supplied  by 
nutrition.  Some  have  even  maintained  that  a  simple  organism 
may  be  physically  attracted  to,  as  well  as  psychically  by,  its 
food.  The  supply  which  the  lifelong  hunger  of  the  protoplasm 
demands,  is  frequently  afforded  in  greater  abundance  than  the 


220 


THE    EVOLUTION    OF    SEX. 


actual  necessities  require.  There  is  a  surplus  for  further  up- 
building after  mere  reparation  has  been  made.  This  surplus 
is  the  condition  of  growth.  Popularly,  but  yet  accurately,  it 
may  be  said  that  growth  or  addition  to  the  capital  of  the 
organism  occurs  when  income  is  in  excess  of  expenditure,  when 
construction  preponderates  over  disruption. 

But  beside  this  familiar  fact,  it  is  necessary  to  place  another 
certainty,  that  of  the  limit  of  growth.  We  may  fairly  call  giants 
a  few  of  the  Protozoa,  such  as  the  large  amoeboid  Pelomyxa, 
some  of  the  gregarines,  and  even  more  markedly  the  extinct 
nummulites,  which  were  sometimes  as  large  as  half-crowns.  So 
an  occasional  alga,  like  Botrydium,  may  swell  out  into  a  large 
single  cell,  and  the  ova  of  animals,  ^.^i^.,  birds,  are  often  greatly 
expanded  by  the  accummulation  of  yolk.  Yet  the  unit  masses 
generally  remain  very  small.     They  have  their  maximum  size. 


Cell-division  at  the  limit  of  growth. 

approximately  constant  for  each  species.  Up  to  this  point  they 
grow,  but  no  further.  The  same,  as  every  one  knows,  is  true 
of  multicellular  animals.  The  size  fluctuates  slightly  according 
to  the  conditions  of  individual  life,  l)ut  the  average  is  strikingly 
constant. 

§  2.  Spence7^s  Theory  of  Growt/i. — The  first  adequate  dis- 
cussion of  growth  is  due  to  Spencer.  He  pointed  out,  that  in 
the  growth  of  similarly  shaped  bodies  the  increase  of  volume 
continually  tends  to  outrun  that  of  the  surface.  The  mass  of 
hving  matter  must  grow  more  rapidly  than  the  surface  through 
which  it  is  kept  alive.  In  spherical  and  all  other  regular  units 
the  mass  increases  as  the  cube  of  the  diameter,  the  surface  only 
as  the  scparc.  Thus  the  cell,  as  it  grows,  must  get  into  physio- 
logical difficulties,  for  the  nutritive  necessities  of  the  increasing 
mass  arc  ever  less  adequately  supplied  by  the  less  rapidly 
increasing  absorbent  surface.     The  early  excess  of  repair  over 


GROWTH    AND    REPRODUCTION. 


221 


waste  secures  the  growth  of  the  cell.  Then  a  nemesis  of  grow- 
ing wealth  begins.  The  increase  of  surface  is  necessarily 
disproportionate  to  that  of  contents,  and  so  there  is  less 
opportunity  for  nutrition,  respiration,  and  excretion.  Waste 
thus  gains  upon,  overtakes,  balances,  and  threatens  to  exceed 
repair.  Suppose  a  cell  to  have  become  as  big  as  it  can  well  be, 
a  number  of  alternatives  are  possible.  Growth  may  cease,  and 
a  balance  be  struck ;  or  the  form  of  the  unit  may  be  altered, 
and  surface  gained  by  flattening  out,  or  very  frequently  by 
outflowing  processes.  On  the  other  hand,  waste  may  continue 
on  the  increase,  and  bring  about  dissolution  or  death ;  while 
closely  akin  to  this,  there  is  the  most  frequent  alternative,  that 
the  cell  divide,  halve  its  mass,  gain  new  surface,  and  restore  the 
balance.  Here,  in  fact,  the  famous  law  of  Malthus  holds  good. 
§  3.  Cell-Division. — What  usually  occurs,  then,  at  the  maxi- 
mum or  limit  of  growth,  is  that  the  cell  divides.  This,  in  its 
simplest  forms,  is  rough  enough  to  suggest  rupture  or  overflow ; 
but  in  the  vast  majority  of  cases  it  is  an  orderly  and  definite 


i/t 


Diagram  of  the  changes  in  the  nucleus  during  cell-division  : — coil  stage 
(a),  the  formation  of  a  double  star  {b,  r,  li),  and  the  recession  of  the 
divided  chromatin  elements  to  opposite  poles  (c)  to  form  the  daughter- 
nuclei  (_/")  of  the  two  daughter-cells.  —  From  Hatschek,  after 
Flemming. 


222 


THE    EVOLUTION    OF    SEX. 


process,  in  which  the  nucleus  plays  an  important  and  probably 
a  controlling  part.  By  a  complicated  series  of  changes,  both 
in  form  and  position,  the  essential  nuclear  elements  group 
themselves  so  as  to  form  the  daughter-nuclei  of  each  product 


Illustrating  thelNIechanisin  of  Cell-Division, — (a)  the 
chromatin  or  essential  elements  of  the  nucleus 
forming  an  "ecjuatorial  plate  "  in  the  one  figure, 
drawn  towards  the  poles  to  form  two  daughter- 
nuclei  in  the  other  ;  (/')  the  almost  '"  muscular  " 
threads ;  (c)  the  protoplasmic  centre  from  which 
these  radiate.  —  From  Boveri. 

of  division.  The  orderliness  and  complexity  of  these  changes 
forbid  any  off-hand  attempt  to  analyse  the  real  physiological 
movement  by  which  the  growth  of  all  multicellular  organisms 
is  effected.     That  attractions  and  repulsions  do  exist  within  the 


GROWTH    AND    REPRODUCTION.  223 

cells  is  certain ;  an  analysis  of  their  precise  nature — the  final 
problem  of  histology— is  still  far  in  the  distance.  We  cannot 
get  within  miles  of  it.  The  problem  has  always  loomed  before 
embryologists  and  histologists, — the  historians  and  mechanicians 
of  the  organism.  Pander,  in  the  first  quarter  of  this  century, 
was  inquiring  into  the  mechanics  of  development,  and  Lotze 
followed  him  with  some  luminous  suggestions.  The  task  has 
been  continued  by  His  and  Rauber ;  while  the  experimental 
investigations  of  O.  Hertwig,  Fol,  Pfliiger,  Born,  Roux,  Schultze, 
Gerlach,  and  others,  have  added  further  stepping-stones. 
Observers  such  as  A^an  Beneden  and  Boveri,  in  their  masterly 
accounts  of  the  morphological  facts,  have  not  left  the  pro- 
blem of  the  actual  dynamics  unessayed ;  while  the  title  of 
Berthold's  book  on  "  Protoplasmic  Mechanics,"  shows  how 
the  biologist  persistently  seeks  the  aid  of  the  student  of 
physics  in  his  endeavour  to  explain  the  architecture  of  the  living 
organism. 

§  4.  Proioplasinic  Restatement. — In  the  above  helpful  sugges- 
tion, Spencer  has  emphasised  the  reasonableness  and  general 
necessity  of  cell-division  at  the  limit  of  growth,  refraining  from 
the  deeper  question  of  the  actual  mechanism  involved.  In 
truth  such  cautious  reserve  must  still  be  maintained,  but  Spencer's 
analysis  admits  of  being  expressed  in  lower  and  more  definite 
terms.  The  early  growth  of  the  cell,  the  increasing  bulk  of 
contained  protoplasm,  the  accumulation  of  nutritive  material, 
correspond  to  a  predominance  of  protoplasmic  processes,  which 
are  constructive  or  anabolic.  The  growing  disproportion  between 
mass  and  surface  must  however  imply  a  relative  decrease  of 
anabolism.  Yet  the  life,  or  general  metabolism,  continues,  and 
this  entails  a  gradually  increasing  preponderance  of  destructive 
processes,  or  katabolisni.  As  long  as  growth  continues,  the 
algebraic  sum  of  the  protoplasmic  processes  must  of  course  be 
plus  on  the  side  of  anabolism,  and  growth  may  be  now  more 
precisely  defined  as  the  outcome  of  the  preponderance  of 
an  anabolic  tendency,  rhythm,  or  bias.  The  limit  of  growth, 
when  waste  has  overtaken  and  is  beginning  to  exceed  the 
income  or  repair,  corresponds  in  the  same  way  to  the  maximum 
of  katabolic  preponderance  consistent  with  life.  The  limit  of 
growth  is  the  end  of  the  race  between  anabolism  and  katabolism, 
the  latter  being  the  winner.  Thus  cell-division  occurs  especi- 
ally at  night,  when  nutrition  is  at  a  standstill,  and  when  there 
is  therefore  a  relative  katabolic  preponderance ;  and  so  explorers 


2  24  THE    EVOLUTION    OF    SEX. 

have  shown  us  that   many  marine  algos  reproduce  during  the 
darkness  of  the  Arctic  winter. 

What  is  true  for  the  cell,  is  true  .for  cell-aggregates. 
Organisms  in  their  entirety  have  very  definite  limits  of  growth. 
Increase  beyond  that  takes  place  at  a  risk,  hence  giant  varia- 
tions are  peculiarly  unstable  and  short-lived.  Or  again,  just  as 
the  single  cell  has  found,  probably  somewhat  pathologically,  a' 
surface-gaining  expedient  in  the  emission  of  mobile  processes, 
so  many  organs,  notably  leaves,  have  struck  a  balance  between 
mass  and  surface  by  becoming  split  up  into  lobes  and  more  or 
less  discontinuous  expansions. 

Spencer  has  laid  great  stress  on  the  importance  of  the 
physiological  capital  with  which  the  organism  begins ;  this 
represents,  in  active  animals  at  least,  the  start  which  their 
anabolism  gets  at  the  outset.  Other  things  equal,  growth  varies 
— {a)  directly  as  nutrition  ;  {!>)  directly  as  the  surplus  of  nutri- 
tion over  expenditure ;  (c)  directly  as  the  rate  at  which  this 
surplus  increases  or  decreases ;  {d)  directly  (in  organisms  of 
large  expenditure)  as  the  initial  bulk  ;  and  {e)  directly  as  the 
degree  of  organisation,— the  whole  series  of  variables  being 
finally  in  close  relation  to  the  doctrines  of  the  persistence  of 
matter  and  conservation  of  energy.  Some  apparent  exceptions 
are  readily  explained.  Thus,  many  plants  seem  to  grow  in- 
definitely, but  they  expend  very  little  energy,  and  have  often 
enormous  surface  area  in  proportion  to  mass.  The  crocodile 
goes  on  slowly  growing,  though  at  a  gradually  diminishing  rate, 
but  it  again  expends  relatively  little  energy  in  proportion  to  its 
high  nutrition.  Birds  which  expend  most  energy,  have  their 
size  most  sharply  defined. 

§  5.  The  A?itit/iesis  between  Growth  and  Multiplication^ 
between  Nutrition  and  Reproduction. — The  life  of  organisms  is 
conspicuously  rhythmic.  Plants  have  their  long  period  of 
vegetative  growth,  and  then  suddenly  burst  into  flower.  Ani- 
mals in  their  young  stages  grow  rapidly,  and  as  the  growth 
ceases  reproduction  normally  begins.  Or  again,  just  as  perennial 
plants  are  strictly  vegetative  throughout  a  great  part  of  the 
year,  but  have  their  stated  recurrence  of  flowers  and  fruit,  so 
many  animals  for  prolonged  periods  are  virtually  asexual,  but 
exhibit  periodic  returns  of  a  reproductive  or  sexual  tide.  In 
some  cases,  such  as  salmon  and  frog,  periods  of  active  and 
preponderant  nutrition  are  followed  by  times  of  fasting,  at  the 
end  of  which  reproduction  occurs.    Foliage  and  fruiting,  periods 


GROWTH    AND    REPRODUCTION.  2  25 

of  nutrition  and  crises  of  reproduction,  hunger  and  love, 
must  be  interpreted  as  life-tides,  ^vhich  will  be  seen  to  be  but 
special  expressions  of- the  fundamental  organic  rhythai  between 
sleep  and  waking,  rest  and  work,  upbuilding  and  expenditure, 
which  are  expressed  on  the  protoplasmic  plane  as  anabolism 
and  katabolism. 

The  common  hydra,  in  abundant  nutritive  conditions, 
produces  numerous  buds,  and  even  these  sometimes  begin 
themselves  to  bear  another  generation.  In  other  words,  we 
may  almost  say,  with  plenty  of  food  the  polypegroias  abundantly, 
so  obviously  is  this  asexual  reproduction  continuous  with  growth. 
A  check  to  the  nutritive  conditions,  however,  brings  on  the  de- 
velopment of  the  sexual  organs  and  the  occurrence  of  sexual 
reproduction.  In  planarian  worms,  the  asexual  multiplication 
of  which  we  have  already  noted,  Zacharias  observed  that 
favourable  nutritive  conditions  were  associated  with  the  forma- 
tion of  asexual  chains,  while  a  check  to  the  nutrition  brought 
about  both  the  separation  and  the  sexual  maturity  of  the  links. 
Rywosch  corroborates  this,  noting  in  Microstoinum  lineare  that 
the  generative  organs  do  not  become  completely  matured  till 
the  individuals  cease  to  be  links  in  a  chain,  and  that  the 
sexuality  is  hastened  by  outside  influences  such  as  checked 
nutrition.  The  gardener  root-prunes  his  apple-tree,  thereby 
checking  nutrition  to  improve  the  yield  of  fruit,  in  other  words, 
to  augment  reproduction.  Reversely,  the  removal  of  repro- 
ductive organs  may  increase  the  development  of  the  general 
"  body  "  both  in  plant  and  animal, — witness  the  castrated  ox, 
capon,  &c.,  or  the  way  in  which  the  gardener  nips  off  the  flower- 
buds  from  his  foliage  plants.  Taking  a  further  step,  we  recall 
the  familiar  and  already  repeated  fact,  that  favourable  nutritive 
and  other  conditions  enable  the  aphides  to  continue  partheno- 
genetic  through  the  summer  months  ;  but  both  for  the  common 
plant-lice  and  for  the  vine-insect  phylloxera,  it  has  been  shown 
that  a  check  to  nutrition  causes  the  parthenogenesis  to  cease, 
and  is  associated  with  the  return  of  sexual  reproduction.  The 
above  instances  are  obviously  not  all  upon  the  same  plane. 
They  illustrate  however,  at  different  levels,  the  same  great  con- 
trast.    It  is  necessary,  however,  to  become  more  precise. 

i^  6.  The  Contrast  between  Gjvwth  and  Repi'oduction  in  the 
Individual. — {ci)  The  Distribution  of  Organs. — The  general 
position  of  the  flow'er  at  the  end  of  the  vegetative  axis  is  so 
obvious  a  fact  that  its  import  tends  to  be  overlooked.    The  end 

p 


226 


THE    EVOLUTION    OF   SEX. 


of  the  axis  is  furthest  from  the  source  of  nutritive  supply ;  with 
exaggeration,  we  might  call  it  the  starvation-point.  There, 
with  kataholic  conditions  tending  relatively  to  predominate,  the 
reproductive  organs  are  situated.  The  flower  occupies  a  kata- 
bolic  position,  and  is  often  the  plant's  dying  effort. 

In  the  tiger-lily,  growth  at  first  tends  to  remain  continuous, 
and  the  base  of  the  bulb  bears  simple  vegetative  buds.     Further 


The  Moonwort  Vern  (/>oiryc/i//n// 
/i^nafc),  showing  the  con- 
trasted frond  (n),  and  fructi- 
fication (./>). — After  Sachs. 


Diagram  of  the  Tiger  Lily,  show- 
ing hulhils  (a)  in  lower  axils, 
and  flower  above. 


up,  however,  where  nutrition  reaches  its  maximum,  the  axils  of 
the  leaves  contain  buds,  which  are  separable  though  still 
asexual.  Finally,  further  up  still,  where  nutrition  is  relatively 
less  active  and  katabolism  is  maximised,  the  formation  of 
flowers  indicates  the  appearance  of  sexual  reproduction. 

In  many  ferns,  the  contrast  between  the  vegetative  and  re- 


GROWTH    AND    REPRODUCTION.  227 

productive  regions  of  the  organism  is  as  marked  as  in  the  flower- 
ing plant.  Thus  the  moonwort  lyBotrychiuni)  and  the  adder's 
tongue  {Ophioglossipn)  have  their  spore-bearing  shoots  standing 
in  conspicuous  antithesis  to  the  leafy  portion,  and  a  similar 
contrast  is  well  seen  in  the  royal  fern  {Osniu?ida)  and  some  of 
its  allies. 

In  animals,  the  contrast  in  position  between  reproductive 
organs  and  the  general  body  is  never  so  marked.  Yet  the 
generally  posterior  position  of  the  organs,  their  frequent  close 
association  with  the  excretory  system,  their  occasional  rupture 
as  external  sacs,  must  not  be  lost  sight  of. 

{b)  The  Contrast  in  the  Individual  Life. —  Growth  during 
youth,  sexual  maturity  at  the  limit  of  growth,  the  continued 
alternation  of  vegetative  and  reproductive  periods,  are  common- 
places of  observation  which  require  no  emphasis.  If  growth 
and  vegetative  increase  are  the  outcome  of  preponderant  ana- 
bolisni,  reproduction  and  sexuality  as  their  antitheses  must  re- 
present the  katabolic  reaction  from  these.  But  anabolism  and 
katabolism  are  the  two  sides  of  protoplasmic  life;  and  the  major 
rhythms  of  their  respective  preponderance  of  these,  give  the 
familiar  antitheses  we  have  been  noting.  These  contrasts  of 
metabolism  represent  the  swings  of  the  organic  see-saw ;  the 
periodic  contrasts  correspond  to  alternate  weightings  or  light- 
enings  of  the  two  sides.  Yet  the  contrast  is  less  than  it  seems. 
In  previous  chapters  we  have  seen  how  growth,  becoming  over- 
growth, turns  into  reproduction ;  and  how  sexual  reproduction, 
dispensing  with  fertilisation,  may  degenerate  till  we  know  it  no 
longer  from  growth.  Reproduction,  moreover,  is  as  primitive 
as  nutrition,  for  not  only  do  hunger  and  love  become  indis- 
tinguishable in  that  equal-sided  conjugation  which  has  been 
curiously  called  "  isophagy,"  but  nutrition  in  turn  is  nothing 
more  than  continual  reproduction  of  the  protoplasm.  Here, 
indeed,  we  have  been  anticipated  by  Hatschek,  who  clearly 
states  the  more  than  verbal  paradox,  that  all  nutrition  is  repro- 
duction. 

§  7.  The  Contrast  betweeti  Asexual  and  Sexual  Repro- 
duction.— In  plenty,  the  hydra  buds  ;  in  poverty,  it  reproduces 
sexually.  In  the  same  way,  the  liverwort  on  the  flower-pot 
bears  its  pretty  cryptogamic  "  flowers "  when  its  exuberant 
growth  and  budding  have  come  to  an  end.  On  rich  soil  the 
plant  has  luxuriant  foliage ;  but  great  abundance  is  the  reverse 
of  conducive  to  the  richest  crop  of  flowers  and  fruit.     Gruber, 


2  28  THE    EVOLUTION    OF    SEX. 

Maupas,  and  others,  have  shown  that  abundant  nutrition  favours 
the  asexual  multipHcation,  /,<?,,  the  division  of  infusorians.  In 
other  words,  the  maximum  size  is  rapidly  reached  wlien  food 
is  abundant,  but  the  conditions  at  the  limit  of  growth  bring 
about  reproduction.  Preponderant  anabolism  leads  up  to  the 
possibility  of  multiplication,  but  we  need  the  onset  of  katabolism 
to  bring  about  the  reproductive  crisis.  Gruber  also  notes,  that 
in  the  very  reverse  of  favourable  conditions,  rapid  division  with 
diminution  of  size  and  resulting  conjugation  sets  in ;  and 
Khawkine  observes  the  occurrence  of  division,  both  at  an 
o[)timum  and  in  famine.  In  both  cases  a  katabolic  crisis  is 
associated  with  reproduction,  though  the  crisis  may  be,  and 
often  is,  preceded  by  an  anabolic  preponderance. 

In  regard  to  a  common  infusorian  {Leucophrys  patida), 
Maupas  observes  that  with  abundant  food  the  ordinary 
fission  continues,  but  with  scanty  nutrition  a  metamorphosis 
occurs,  followed  by  six  successive  divisions,  which  have  for 
their  end  conjugation.  That  is  to  say,  w^e  have  positive  proof 
that  in  these  lowest  organisms,  katabolic  conditions  determine 
the  beginning  of  sexual  reproduction,  a  matter  of  no  small 
importance  to  the  evolutionist.  Generalising,  M.  Maupas 
concludes,  that  the  reproductive  power  of  ciliated  infusorians 
depends,  (i)  on  the  quality  and  quantity  of  the  food;  (2)  on 
the  temperature ;  (3)  on  the  alimentary  adaptation  of  the 
buccal  organs.  He  also  demonstrates,  that  with  a  vegetarian 
diet  their  rate  of  asexual  reproduction  is  much  less,  and  the 
size  smaller.  Taking  these  facts,  along  with  his  important 
demonstration  that  the  life  of  ciliated  infusorians  runs  in  cycles 
of  asexual  reproduction,  necessarily  interrupted  (if  the  life  of 
the  species  is  to  continue)  by  conjugation  or  sexual  repro- 
duction, we  again  reach  the  general  conclusion,  that  anabolic 
conditions  favour  asexual  reproduction,  rather  than  sexual  ;  and 
that  while  preponderant  anabolism  is  the  necessary  condition 
of  the  overgrowth  which  makes  the  asexual  reproduction 
possible,  the  onset  of  katabolic  preponderance  is  necessary  to 
the  act  itself 

Semper  quotes  an  interesting  observation  by  Strethill  Wright, 
unfortunately  somewhat  vague,  that  certain  polyps  multiply 
abundantly  in  the  dark  by  buds,  while  in  the  light,  and  with 
insufficient  supplies  of  food,  they  bring  forth  sexual  individuals 
or  meduscE.  More  })recise  is  the  fact  already  cited  from 
Zacharias,    that    the    spontaneous    asexual     multiplication    of 


GROWTH    AND    REPRODUCTION.  229 

planarians  went  on  apace  when  the  food  supply  was  copious 
(anaboHc  condition),  but  if  the  amount  of  food  was  reduced 
or  altogether  withdrawn  (katabolic  condition)  the  asexual  re- 
production completely  ceased.  Bergendal  reports,  that  in  the 
transverse  division  of  another  planarian  worm  {Bipalimti)^  the 
severed  links  were  all  sexually  immature ;  and  the  results  of 
Rywosch  demonstrate  the  same  antithesis  between  the  sexual 
and  the  asexual  process. 

In  the  same  way,  sexual  reproduction  is  contrasted  with  its 
degenerate  expression  in  parthenogenesis.  The  conditions  of 
the  latter  in  aphides  and  phylloxera  are  demonstrably  anabolic, 
the  normal  sexual  process  recurs  with  the  periodic  return  of  hard 
times,  or  in  relatively  katabolic  conditions.  In  the  lower  crus- 
taceans, a  similar  contrast  of  conditions  has  also  been  observed. 


/ 


Pollen  Grain  ;  a,  the  two  nuclei ;  I',  the  general  protoplasm 
c,  the  outer  wall. — From  Carnoy. 

It  is  again,  on  the  present  view,  readily  intelligible  why  in 
the  exceptionally  favourable  anabolic  environment  of  bacteria 
and  many  parasitic  fungi  sexual  reproduction  should  be  absent. 
Marshall  Ward  has  pointed  out,  that  the  more  intimate  the 
degree  of  parasitism  or  saprophytism,  the  more  degenerate  the 
sexual  reproduction.  The  greater  the  anabolism,  in  other 
words,  the  more  growth  and  the  less  sexuality.  That  such 
comparatively  complex  organisms  can  continue  their  asexual 
reproduction,  dispensing  altogether  with  the  acknowledged 
stimulus  of  fertilisation,  may  probably  be  at  least  partially 
explained  on  the  assumption  that  the  abundant  waste  products 
of  the  host  act  as  extrinsic  stimuli. 

On  this  view,  moreover,  alternation  of  generations  loses 
much  of  its  uniqueness.     The  contrast  between  the  vegetative 


230  THE    EVOLUTION    OF    SEX. 

asexual  hydroid  or  hydra-tuba,  and  the  active  sexual  medusoid 
or  jelly-fish,  is  very  marked.  So  too,  on  a  higher  plane,  the 
vegetative  spore-producing  fern-plant  stands  opposed  to  the  less 
nutritive  sexual  prothallus.  The  alternation  is  but  a  rhythm  of 
large  amplitude  between  anabolic  and  katabolic  preponderance. 

What  is  so  marked  in  the  alternation  is  only  a  special- 
isation of  the  reproductive  or  sexual  parts  of  the  organism  as 
against  the  growing  or  asexual  ones, — a  specialisation  which 
becomes  exaggerated  into  separate  existences,  each  dominated 
by  its  own  physiological  bias. 

In  the  fern  or  flowering  plant  the  vegetative  or  asexual 
existence  has  preponderated,  and  this  is  entirely  consistent 
with  the  characteristic  passivity  of  plants.  This  is  emphatically 
their  line  of  development ;  but,  be  it  observed,  that  though 
in  the  flowering  plants  the  nutritive  generation  has  dwarfed, 
and  included  the  sexual,  which  seem  indeed  to  be  mere 
organs, — the  pollen-grain  and  embryo  sac, — yet  it  is  through 
and  for  these  that  we  have  all  the  glory  of  the  flower  (see  fig. 
p.  211).  In  animals,  with  their  emphatically  active  line  of  de- 
velopment, the  reproductive  generation  is  the  higher ;  and  in 
the  higher  forms  the  separate  asexual  existence  is  wholly  lost. 


GROWTH    AND    REPRODUCTION.  23 1 


SUMMARY. 

1.  Growth  is  characteristic  of  living  organisms,  though  analogous  pro- 
cesses occur  at  the  inorganic  level.  Hunger  is  an  essential  characteristic  of 
living  matter.  As  certain  as  the  fact  of  growth,  is  the  definiteness  of  its 
limit  alike  for  cell  and  for  organism. 

2.  Spencer  has  analysed  the  limit  of  growth,  in  terms  of  the  continual 
tendency  that  increase  of  mass  must  have  to  outrun  increase  of  surface. 

3.  Cell-division  at  the  limit  of  growth,  at  the  maximum  or  optimum  of 
size,  restores  the  balance  between  mass  and  surface.  The  actual  mechanics 
of  the  process  are  at  present  beyond  analysis. 

4.  Spencer's  analysis  may  be  restated  in  protoplasmic  terms.  Growth 
expresses  the  preponderance  of  anabolism  ;  increase  of  mass,  with  less 
rapid  increase  of  nutritive,  respiratory,  and  excretory  surface,  involves  a 
relative  predominance  of  katabolism.  The  limit  of  growth  occurs  when 
katabolism  has  made  up  upon  anabolism,  and  tends  to  outstrip  it.  What 
is  true  of  the  unit,  applies  also  to  the  entire  multicellular  organism. 

5.  Throughout  organic  life  there  is  a  contrast  or  rhythm  between 
growth  and  multiplication,  between  nutrition  and  reproduction,  corre- 
sponding to  the  fundamental  organic  see-saw  between  anabolism  and 
katabolism. 

6.  This  contrast  may  be  read  in  the  distribution  of  organs,  in  the  periods 
of  life,  and  in  the  different  grades  of  reproduction.  Yet  nutrition  and 
reproduction  are  fundamentally  nearly  akin. 

7.  The  contrasts  between  continuous  growth  and  discontinuous  multi- 
plication, between  asexual  and  sexual  reproduction,  between  partheno- 
genesis and  sexuality,  between  alternating  generations,  are  all  different 
expressions  of  the  fundamental  antithesis. 


LITERATURE. 
Spencer,  Principles  of  Biology  ;  and  ILeckei.,  Generelle  Morphologic. 


CHAPTER  XVII. 

Theory  of  Reproduction. 

§  I.  The  Essential  Fact  in  Reproduction. — In  the  foregoing 
chapters,  the  facts  involved  in  the  different  forms  of  repro- 
duction have  been  analysed  apart,  and  separately  discussed. 
Male  and  female  organisms  have  been  interpreted  as  relatively 
katabolic  and  anabolic ;  the  origin  of  sex,  in  the  individual 
and  in  the  race,  has  been  traced  back  to  the  preponderance  of 
anabolic  or  katabolic  conditions ;  the  ultimate  sex -elements 
were  seen  to  exhibit  the  same  contrast  in  its  most  concentrated 
expression  ;  fertilisation  was  regarded  as  a  katabolic  stimulus  to 
an  anabolic  cell,  and  on  the  other  side,  of  course,  as  an  anabolic 
renewal  to  a  katabolic  cell,  as  well  as  the  union  of  opposed 
hereditary  characteristics.  Only  by  a  separation  of  the  problem 
of  "sexual  reproduction"  into  its  component  problems  can  the 
solution  be  reached.  Sexual  reproduction  is  like  a  complex 
musical  chord  in  the  organic  life,  combining  several  elements,  all 
of  which,  however,  admit  of  the  same  fundamental  analysis. 
Two  problems  remain, — the  psychical  aspect  of  the  process  ; 
and  the  import  of  that  common  feature  of  all  reproduction,  the 
separation  of  part  of  the  parent  organism  to  start  a  fresh  life. 
The  latter  forms  the  subject  of  the  present  chapter. 

§  2.  Arginnent  from  the  Beginnings  of  Reproduction.  — 
Leconte  and  others  have  pointed  out  that  reproduction  really 
begins  with  the  almost  mechanical  breakage  of  a  unit  mass  of 
living  matter,  which  has  grown  too  large  for  successful  co-ordi- 
nation. Reproduction,  in  fact,  begins  as  rupture.  Large  cells 
beginning  to  die,  save  their  lives  by  sacrifice.  Reproduction  is 
literally  a  life-saving  against  the  approach  of  death.  Whether  it 
be  the  almost  random  rupture  of  one  of  the  more  primitive 
forms  such  as  Schizoge?ies,  or  the  overflow  and  separation  of 
multiple  buds  as  in  Arcella.^  or  the  dissolution  of  a  few  of  the 
infusorians,  an  organism,  which  is  becoming  exhausted,  saves 
itself,  and  multiplies  in  reproducing.     In  some  cases,  reproduc- 


THEORY    OF    REPRODUCTION. 


233 


tion  is  effected  by  outflowing  processes  of  the  cell,  which  have 
gone  a  little  too  far.  Now,  such  primitive  forms  of  multiplica- 
tion, gradually  becoming  more  definite,  express  a  predominant 
katabolism  in  the  unit  mass.  Reproduction  in  its  simplest 
forms  is  associated  with  a  katabolic  crisis. 

§    3.    Argument  from    Cell- Division.  —  Most    unicellular 
organisms  reproduce  by  cell-division,  and  this  is,  of  course,  a 


Division  of  an  Animal  Cell,  showing  the  nucleus  («)  in  process  of  forming 
two  daughter-nuclei,  showing  also  the  protoplasmic  network  (J').— 
From  Carnoy. 

precedent  of  reproduction  in  multicellular  organisms,  whether 
they  multiply  by  asexual  budding  or  by  differentiated  sex- 
elements.  But  in  the  preceding  chapter,  following  Spencer,  we 
have  emphasised  the  connection  between  division  and  a  katabolic 
predominance  within  the  cell.  A  constructive  period  may 
precede,  but  a  disruptive  climax  attends  the  division.  So  far 
then  as  reproduction  is  either  wholly  included  in  the  process  of 


234  THE    EVOLUTION    OF    SEX. 

cell-division,  or  has  this  as  its  necessary  precedent,  it  is  associated 
with  a  katabolic  crisis. 

§  4.  Argument  from  the  Gradations  between  Asexual  Sever- 
ance of  Parts  and  the  Liberation  of  Special  Sex- Cells. — Discuss- 
ing asexual  reproduction,  we  have  noticed  that  some  worm-types 
break  into  two  or  more  parts,  which  start  new  individuals.  That 
some  nemerteans  normally  break  up  into  pieces,  as  they  do  in 
the  feverish  anxiety  of  capture,  is  most  probable ;  and  this  is 
certainly  the  case  in  certain  annelids.  From  a  syllid,  which 
sets  free  a  sexual  individual,  the  overgrowth  of  an  asexual  parent, 
to  one  which  liberates  a  series  of  joints,  or  even  a  single  joint, 
bearing  reproductive  elements,  is  but  a  slight  step.  From  the 
last  case,  to  the  rupture  which  liberates  sex-elements,  is  again 
only  a  slight  advance.  A  similar  series  is  well  illustrated  among 
the  Hydromedusce.  The  breakage  or  thinning  away  which  sets 
a  large  portion  free  is  a  katabolic  process,  in  a  sense  a  local 
death.  The  gentleness  of  the  gradient  warrants  us  in  concluding 
that  the  liberation  of  sex-cells,  in  its  earlier  expressions  at  least, 
is  associated  with  a  local  or  with  a  general  katabolic  crisis. 

§  5.  Argumefit  f'om  the  Close  Connectio?i  betiveen  Reproduc- 
tion and  Death. — Without  going  back  to  primitive  disintegra- 
tions, or  the  asexual  severance  of  more  or  less  large  portions, 
we  may  point  further  to  the  close  connection  between  reproduc- 
tion and  death,  even  when  the  former  is  accomplished  by 
specialised  sex-cells.  We  shall  presently  discuss  at  greater 
length  this  nemesis  of  reproduction,  but  it  is  important  here  to 
emphasise  that  the  organism  not  unfrequently  dies  in  continu- 
ing the  life  of  the  species.  In  some  species  of  the  primitive 
annelid  Polygordius,  the  mature  females  die  in  liberating 
the  ova.  At  a  very  different  level,  the  gemmules  of  the 
common  fresh-water  sponge  are  formed  in  the  decay  of  the 
asexual  adult,  while  even  the  sexual  summer  forms,  especially 
the  males,  are  peculiarly  unstable  and  mortal.  The  whole 
history  of  this  form  seems  a  continuous  rhythm  between  life 
and  growth  on  the  one  hand,  and  death  and  reproduction  on  the 
other.  Or  again,  the  flowering  of  phanerogams  is  often  at  once 
the  climax  of  the  life  and  the  glory  of  death.  In  his  ingenious 
essay  on  the  origin  of  death,  Goette  has  well  shown  how  closely 
and  necessarily  bound  together  are  the  two  facts  of  reproduction 
and  death,  which  may  be  both  described  as  katabolic  crises. 

§  6.  Argument  from  E7ivironmental  Conditions  wJiich  favour 
Reproduction. — The  rhythm  between   nutrition  and   reproduc- 


THEORY    OF    REPRODUCTION.  235 

tion,  or  between  growth  and  multiplication,  has  been  as  it  were 
the  refrain  of  the  preceding  pages.  This  "organic  see-saw" 
is  determined  by  the  very  constitution  of  the  organism ;  in  other 
words,  it  expresses  the  fundamental  characteristic  of  living 
matter.  It  is  an  incomplete  conception,  however,  unless  it  be 
remembered  that  about  this  "  organic  see-saw  "  there  blows  the 
wind  of  the  environment,  swaying  it  now  to  one  side,  now  to 
the  other.  It  is  important  therefore  to  illustrate  how  the  play 
of  external  conditions  accelerates  or  retards  the  reproductive 
function. 

The  influence  of  heat  upon  the  reproductive  powers  of 
infusorians  has  been  carefully  investigated  by  Maupas.  The 
higher  the  temperature  up  to  a  certain  limit,  the  faster  do  these 
organisms  reproduce.  In  favourable  nutritive  conditions,  Stylo- 
nichia  pustulata  divides  once  in  twenty-four  hours  at  a  tem- 
perature of  7°  to  10°  C,  twice  at  io°  to  15°,  thrice  at  15°  to  20°,  four 
times  at  20°  to  24,  and  five  times  at  24°  to  27°  C.  Illustrating  the 
rapid  rate  of  increase,  Maupas  notes  in  the  same  paper,  that  at 
a  temperature  of  25''  to  26^  C,  a  single  Stylonichia  would  in  four 
days  have  a  progeny  of  a  million,  in  six  days  of  a  billion,  in 
seven  and  a  half  days  of  a  hundred  billions  !  In  six  days  the 
family  would  weigh  one  kilogramme,  and  in  seven  and  a  half 
days  one  hundred  kilogrammes. 

The  action  of  heat  may  be  twofold ;  up  to  a  certain  limit  it 
quickens  development  and  the  general  life,  favouring  asexual 
reproduction  and  parthenogenesis  rather  than  the  sexual  pro- 
cess ;  beyond  that  limit  of  comfortable  warmth,  so  variable  for 
different  animals,  it  may  induce  a  feverish  habit  of  body,  and 
hasten  reproductive  maturity  and  sexual  reproduction.  In  other 
words,  heat  may  in  some  cases  favour  anabolism,  in  others 
katabolism.  It  is  intelligible  enough  to  find  increased  heat 
sometimes  associated  with  increased  asexual  reproduction, 
sometimes  with  accelerated  sexuality.  Instances  of  both  may 
be  gathered  from  Semper's  "  Animal  Life,"  the  classical  work 
on  the  influence  of  the  environment  upon  the  organism. 

Maupas  supplies  another  vivid  illustration  of  a  yet  more 
important  environmental  influence,  that  of  food.  In  another 
ciliated  infusorian  {Leiicophrys),  so  long  as  food  is  abundant, 
fission  obtains  ;  but  when  food  grows  scanty,  there  is  a  metamor- 
phosis without  encystation,  followed  by  six  successive  divisions. 
These  are  effected,  however,  "without  vegetative  growth,  and 
have  for  their  final  object  not  multiplication  but  conjugation." 


236  THE    EVOLUTION    OF    SEX. 

In  Other  words,  abundant  food  is  associated  with  asexual  repro- 
duction ;  a  check  to  the  nutrition  brings  about  the  sexual  process. 
Maupas  gives  a  vivid  numerical  statement  of  the  stimulus  to 
reproduction  by  a  sudden  check  to  the  nutrition.  I.eucophrys 
at  a  temperature  of  20°  C,  in  richly  nutritive  conditions,  will 
give  rise  to  sixteen  thousand  three  hundred  and  eighty-four 
individuals  in  three  days ;  but  if  the  food  be  then  suppressed, 
this  large  number  will  in  a  few  hours  be  multiplied  by  sixty -four, 
resulting  in  a  total  of  one  million  forty-eight  thousand  five 
hundred  and  seventy-six  individuals  ! 

From  cases  already  cited,  which  may  be  multiplied  by  con- 
sulting Semper's  "Animal  Life,"  supplemented  by  a  summary 
of  more  recent  researches  by  one  of  ourselves,  the  general  con- 
clusions may  be  drawn, — (a)  That  heat  increases  reproduction, 
either  directly  or  as  the  result  of  a  preliminary  acceleration  of 
growth ;  (/^)  That  increased  food  will,  of  course,  favour  growth, 
but  reproduction  may  follow  all  the  more  markedly  as  an 
exaggerated  nemesis  ;  (r)  That  checks  to  nutrition,  especially 
in  the  form  of  sudden  scarcity,  will  favour  sexual  reproduction. 
The  clearest  result  of  all  is,  that  a  sudden  katabolic  change 
favours  reproduction,  especially  in  its  sexual  form.  Anabolic 
conditions  favour  reproduction  indirectly ;  the  reverse  condi- 
tions have  a  direct  influence ;  in  both  cases,  reproduction  is  the 
expression  of  a  katabolic  crisis. 

7.  Conclusion. — Primitively,  then,  reproduction  was  a  kata- 
bolic rupture  of  a  mass  of  protoplasm.  This  becomes  more 
definite  in  cell-division  of  various  kinds,  tending  ever  to  occur 
at  the  limit  of  growth  when  waste  has  made  up  on  repair,  or  in 
katabolic  conditions  due  to  the  environment.  In  multicellular 
animals,  anabolic  conditions  favour  overgrowth  ;  a  check  to  this 
brings  about  discontinuous  asexual  reproduction.  With  in- 
creasing diff'erentiation,  the  asexual  multii)lication  is  replaced  by 
the  liberation  of  special  sex-cells,  by  which  the  life-saving  and 
life-continuing  sacrifice  is  rendered  less  costly.  Just  as  asexual 
reproduction  occurs  at  the  limit  of  growth,  so  a  check  to  the 
asexual  process  is  often  seen  to  involve  the  appearance  of  the 
sexual,  which  is  thus  still  further  associated  with  katabolic  pre- 
ponderance. This  is  confirmed  by  the  contrasts  observed  in 
alternation  of  generations,  where  the  two  processes  in  varying 
degrees  of  distinctness  persist  in  the  life-history  of  the  same 
organism.  Corroboration  is  again  afforded  by  the  association 
of  sexual  reproduction  with  sundry  environmental  checks  of  a 


THEORY    OF    REPRODUCTION. 


237 


katabolic  character.  And  thus  the  opposition  between  nutri- 
tion, which,  after  hfe  and  death,  is  the  most  obvious  antithesis 
in  nature,  admits  of  being  more  precisely  restated  in  the  thesis, 
that  as  a  continued  surplus  of  anaboHsm  involves  growth,  so  a 
relative  preponderance  of  katabolism  necessitates  reproduction. 
Or  this  may  be  summed  up  once  more  in  our  fundamental 
diagrams  : — 

SUM    OF    FUNCTIONS. 


Nutrition.  Reproduction. 


Anabolism.     Katabolism.     Female.         Male. 


238  THE    EVOLUTION    OF    SEX. 


SUMiMARY. 

1.  The  essential  fact  in  reproduction  is  the  separation  of  part  of  the 
parent  organism  to  start  a  fresh  life. 

2.  Reproduction  begins  with  rupture, — a  katabolic  crisis. 

3.  Cell-division,  which  sometimes  sums  up,  and  is  always  associated 
with,  the  act  of  reproduction,  occurs  at  a  katabolic  crisis. 

4.  The  gradations  between  discontinuous  asexual  multiplication  and 
ordinary  sexual  reproduction,  show  a  lessening  of  the  sacrifice  ;  but  all  de- 
mand a  disruption,  or  a  katabolic  preponderance, 

5.  From  first  to  last  reproduction  is  linked  to  death. 

6.  Environmental  conditions  of  a  katabolic  character  favour  sexual  re- 
production. 

7.  General  conclusion, — a  relative  preponderance  of  katabolism  neces- 
sitates reproduction. 


LITERATURE. 

Geddes,  p. — Theory  of  Growth,  Reproduction,  Sex,  and  Heredity.    Proc. 

Roy.  Soc.  Edin.  1886. 
H^:cKEL. — Generclle  Morphologic.      1866. 
vSpencer. — Principles  of  Biology.      1866. 
Semper.— Animal  Life.     Int.  Sci.  Series.      1881. 
Thomson. — "  vSynthetic  Summary  of  the  Influence  of  the    Environment 

upon  the  Organism."     Proc.  Roy.  Phys.  Soc.  Edin,      1887. 


CHAPTER    XVIII. 

Special  Physiology  of  Sex  and  Reproduction. 

It  is  no  part  of  our  purpose  to  discuss  in  detail  the  physiology 
of  sexual  and  reproductive  functions.  The  fundamental  physi- 
ology of  the  essential  functions  has  been  the  subject  of  preced- 
ing chapters ;  the  details  will  be  found  in  the  standard  works 
on  Physiology,  Botany,  and  Zoology.  For  the  sake  of  com- 
pleteness, however,  it  is  necessary  to  take  a  brief  survey  of  some 
of  the  facts,  which  are  in  themselves  of  supreme  importance, 
and  which  further  elucidate  the  general  biology  of  the  subject. 

§  IVeismann's  Theory  of''''  Contitiiiity  of  the  Gei'm-Flas?na." — 
Thanks,  esi)ecially  to  Weismann,  the  view  that  ordinary  cells  of 
the  "  body  "  become  at  a  certain  epoch  changed  into  special 
reproductive  cells,  may  now  be  put  aside  as  exceedingly  im- 
probable. In  a  minority  of  cases,  already  quoted,  the  repro- 
ductive cells,  or  the  rudiments  of  sexual  organs,  are  demonstrably 
set  apart  at  an  early  stage,  before  the  differentiation  of  the 
embryo  has  proceeded  far.  They  thus  include  some  of  the 
original  capital  of  the  fertilised  parent  ovum  intact,  they  con- 
tinue the  protoplasmic  tradition  unaltered,  and,  when  liberated 
in  turn,  they  naturally  enough  develop  as  the  parent  ovum  did. 
Following  out  this  important  fact,  various  naturalists  have 
reached  the  conception  of  a  continuous  necklace-like  chain  of 
sex-cells  from  generation  to  generation, — a  continuous  chain 
upon  which  the  mortal  individual  organisms  arise  and  drop 
away,  like  so  many  separate  and  successive  pendents. 

But  in  the  majority  of  cases,  such  a  conception,  as  Weismann 
has  justly  insisted,  gives  a  false  simplicity  to  the  facts.  A  chain 
of  insulated  sex-cells,  connecting  the  parental  fertilised  ovum 
with  the  germ-cells  which  develop  into  offspring,  is,  so  far  as 
we  yet  know,  only  rarely  demonstrable.  In  other  words,  the 
rudiments  of  the  reproductive  organs  often  appear  at  a  relatively 
late  stage  in  the  development.  Where  do  they  come  from  ? 
Are  somatic,  or  ordinary  body-cells  modified  into  reproductive 


240 


THE    EVOLUTION    OF    SEX. 


elements  ?  Weismann's  answer  is  a  decided  negative.  Although 
no  continuous  chain  of  germ-like  cells  is  demonstrable,  there  is 
a  strict  continuity  oi  (j^Qx\T\-plas}na.  Part  of  the  double  nucleus 
of  the  fertilised  ovum  keeps  its  characteristics  unaltered,  in  spite 
of  manifold  divisions  persists  intact,  and  is  finally  established 
in  the  rudiment  of  reproductive  organs.  Or  in  other  words, 
those  cells  in  which  the  original  germ-plasma  most  predominates 
become  the  reproductive  cells.  To  quote  Weismann's  own 
words,  "  In  each  development  a  portion  of  the  specific  germ- 
plasma  which  the  parental  ovum  contains,  is  not  used  up  in 
the  formation  of  the  offspring,  but  is  reserved  unchanged  to 


The  cluoiiiatin  elements  of  the  nuclei  in  coil  («),  douljle  star  (/'),  and  almost 
divided  stages  (c). — After  Pfitzner. 


form  the  germ-cells  of  the  following  generation."  In  short,  con- 
tinuity is  kept  up  by  the  plasma  of  nuclei,  rather  than  by  a  chain 
of  cells.  It  will  be  observed,  of  course,  that  while  early  insul- 
ation of  definite  germ-cells  is  a  demonstrable  fact,  to  be  seen 
in  a  few  cases,  though  perhaps  of  wider  occurrence  than  we 
know  of,  the  continuity  of  germ-plasma  is  strictly  an  hypothesis. 
This  being  so,  reproductive  maturity  may  be  defined  as  the 
period  when  the  reproductive  cells  (bearing  the  inherited  capital 
of  germ-plasma)  have  established  themselves  to  that  degree 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       241 

that  they  can  start  fresh  organisms,  and  have  multiplied  to  an 
extent  which  in  most  cases  makes  their  liberation  a  physiological 
necessity.  In  the  lower  animals,  the  maturity  of  the  sexual 
functions  is  often  as  slightly  marked  as  the  liberation  of  the 
elements  is  passive  and  random.  In  slightly  differentiated 
organisms,  like  sponges,  there  is  little  reason  to  suppose  that  the 
distinction  between  cells  preponderating  in  germ-plasma  and 
the  ordinary  cells  of  the  body  is  much  marked.  Nor  in  such 
cases  is  the  anarchic  opposition  between  body  and  reproductive 
cells  at  all  emphatic,  especially  as  regards  the  female  cells.  It 
is  only  as  the  differentiation  increases,  as  the  contrast  between 
body-cells  and  sex-cells  becomes  emphasised,  as  the  asexual 
mode  of  getting  rid  of  surplus  wanes,  that  the  typical  liberation 
of  sex-elements  which  marks  sexual  maturity  becomes  a  striking 
fact  in  the  life.  That  the  male-cells  are  always  more  anarchic, 
usually  mature  before  the  female  elements,  and  even  in  plants, 
and  in  such  passive  animals  as  a  sponge  or  a  hydra,  burst  from 
the  organism,  while  the  female  cells  remain  in  situ,  is  quite  con- 
sonant with  their  predominantly  katabolic  character. 

§  2.  Sexual  Maturation. — I'he  maturation  of  the  sexes  not 
only  acquires  increasing  definiteness  in  the  higher  forms,  but 
becomes  associated  with  various  characteristic  accompaniments. 
The  profound  reaction  of  reproductive  maturity  upon  the  whole 
system  is  best  marked  in  birds  and  mammals,  and  perhaps 
most  of  all  in  man.  Thus  in  a  young  male  bird,  the  circulation 
in  the  testes  is  greatly  increased,  and  these  organs  increase 
greatly  in  size  and  weight,  and  commence  to  develop  sperma- 
tozoa. Meanwhile  the  "secondary  sexual  characters"  of  the 
adult — gayer  plumage  for  alluring  the  female,  or  weapons  for 
contest  with  other  males — make  their  appearance,  the  voice  and 
note  may  alter,  and  a  marked  increase  of  strength  and  courage 
may  appear.  Among  mammals,  the  changes  are  of  similar 
order,  the  secondary  sexual  characters  of  course  differing  in 
detail.  The  minor  changes  at  puberty  in  man  associated  with 
the  commencement  of  spermatogenesis,  are  (besides  the  reflex 
excitation  of  erection  due  to  distension  of  the  seminal  vesicles, 
and  the  more  or  less  periodic  expulsion  of  their  contents  during 
sleep)  the  growth  of  hair  on  the  pubic  region  and  later  on  the 
lower  part  of  the  face,  and  the  rapid  modification  of  the 
laryngeal  cartilages  and  the  lengthening  of  the  vocal  chords,  so 
rendering  the  voice  harsh  and  broken  during  the  change,  and 
ultimately  deepening   it   by    about   an    octave.     The  marked 

Q 


242  THE    EVOLUTION    OF    SEX. 

Strengthening  of  bones  and  muscles,  and  the  profound  psychical 
changes  which  accompany  the  whole  series  of  processes,  are  also 
familiar. 

In  higher  vertebrates,  the  sexual  maturity  of  the  female  is 
marked  by  a  cellular  activity  within  the  ovary,  not  less  remark- 
able than  that  in  the  testes.  Associated  therewith  are  minor 
but  often  very  important  characteristics,  such  as  the  increased 
mammary  development  in  mammalia.  In  some  of  the  lower 
animals,  such  as  certain  marine  annelids,  the  ova  become  so 
numerous  that  their  disruption  or  liberation  is  in  great  part  a 
mechanical  necessity.  The  same  might  be  said  of  fishes, 
reptiles,  and  birds.  At  the  same  time  the  enlargement  and 
escape  of  the  ova  are  doubtless  expressions  of  a  normal  cellular 
rhythm,  of  which  hints  are  given  in  the  frequent  passage  from 
an  amoeboid  to  an  encysted  phase,  in  the  occasional  relapse  to 
the  former,  and  in  the  fatty  degeneration  or  death  of  ova  which 
have  not  accomplished  their  destiny. 

The  primitive  ova  of  vertebrates  lie  in  clusters  in  the  substance  or 
stroma  of  the  organ,  and  are  produced  from  the  essential  germinal 
epithelium.  Only  a  minority,  however,  grow  into  genuine  ova  ;  others,  of 
smaller  size,  form  a  nutritive  sheath  or  follicle  around  them.  In  mammals, 
each  follicle  forms  a  cavity  containing  a  fluid.  Into  this  the  ovum,  sur- 
rounded by  a  mass  of  follicle  cells,  projects.  When  mature,  the  follicle  with 
its  contained  ovum  has  attained  a  superficial  position.  By  the  bursting  of 
the  ripe  follicle  the  ovum  is  expelled,  and  passes  into  the  approximated  and 
ciliated  upper  end  of  the  oviduct  or  Fallopian  tube.  The  rupture  of 
blood-vessels  in  the  substance  of  the  ovary  fills  up  the  Graafian  follicle  with 
blood.  The  white  corpuscles  form  a  framework  resembling  connective 
tissue,  in  which  the  solids  and  corpuscles  of  the  blood  serum,  with  colour- 
ing matter  derived  from  the  hemoglobin  of  the  latter,  are  retained.  The 
whole  constitutes  the  "corpus  luteum,"  which,  should  pregnancy  occur, 
may  persist  and  undergo  further  retrogressive  changes,  or  otherwise 
gradually  disappear. 

As  to  the  direct  causes  of  this  process  of  ovulation  there  is  some 
difference  of  opinion.  The  congestion  of  the  blood-vessels  of  the  ovary, 
its  own  internal  turgidity,  a  slight  contractility  of  its  stroma,  have  been 
regarded  as  determining  factors.  The  process  seems,  however,  rather  to 
depend  upon  the  growth  and  turgescence  of  the  individual  follicle.  The 
question  of  the  relation  of  ovulation  to  the  process  of  copulation  in  the 
higher  animals  has  also  been  much  discussed.  Though  we  certainly  know 
that  ovulation  is  of  regular  occurrence  whether  fecundation  takes  place  or 
not,  it  seems  that  in  many  cases  copulation  is  speedily  followed  by  the 
liberation  of  an  ovum  ;  nor  is  it  difficult  to  see  how  the  profound  nervous 
and  circulatory  excitement  associated  with  the  former  process  might 
accelerate  the  bursting  of  a  follicle.  Leopold  has  conclusively  shown, 
however,  that  ovulation  may  also  long  precede  impregnation. 

Since  the  oviduct,  unlike  its  male  counterpart,  is  not,  in  the  vast 
majority  of  vertebrates,  continuous  with  its  associated   organ,  it  is  often 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       243 

difficult  to  see  how  the  ova  once  liberated  into  the  body-cavity  find  their 
way  safely  into  the  small  opening  of  the  duct.  In  the  frog,  however, 
tracts  of  the  peritoneal  epithelium  become  ciliated,  so  propelling  the  ova 
in  the  right  direction.  In  reptiles,  birds,  and  mammals  the  open  end  of 
the  oviduct  is  widened,  fringed,  and  ciliated,  and  lies  close  to  or  even 
touching  the  ovaiy  ;  muscular  fibres  too  are  present,  and  more  or  less 
active  movements  of  this  cilated  end  over  the  ovarian  surface  have  been 
alleged  to  occur.  The  oviduct  once  reached,  the  downward  progress  of 
the  ovum  is  ensured  by  the  cilia  of  the  epithelial  lining,  and  probably  also 
by  peristaltic  movements  of  its  muscular  coat. 

There  is  no  doubt  that  the  advent  of  sexual  maturity  varies 
with  environmental  conditions  of  dimate,  food,  and  the  like. 
Broadly  speaking,  sexuality  becomes  pronounced  as  growth 
ceases.  Especially  in  higher  organisms,  a  distinction  must 
obviously  be  drawn  between  the  period  at  which  it  is  possible 
for  males  and  females  to  unite  in  fertile  sexual  union,  and  the 
period  at  which  such  union  will  naturally  occur  or  will  result  in 
the  fittest  offspring.  In  the  lower  animals,  where  the  individual 
life  is  usually  shorter,  sexual  maturity  is  more  rapidly  attained, 
though  we  find  cases  such  as  that  of  the  fluke  {^Polystomwti)  so 
commonly  present  in  the  bladder  of  the  frog,  where  maturity  of 
the  reproductive  organs  does  not  occur  for  several  (three)  years, 
and  maturity  of  growth  for  some  years  afterwards.  In  cestode 
parasites,  the  bladder-worm  stage  remains  indefinitely  asexual, 
until  in  fact  the  stimulus  of  a  new  host  admits  of  the  develop- 
ment of  the  sexual  tapeworm.  In  plants,  reproductive  maturity 
sets  in  at  various  ages ;  thus  we  have  all  gradations,  at  the 
one  extreme  our  characteristically  short-lived  but  magnificent 
annuals,  then  the  biennials,  and  from  these  to  a  maturation  at 
still  longer  date,  as  in  the  well-known  case  of  the  American 
aloe  {^Aloe  ame7'icand)^  which  even  in  Mexico  takes  from  seven 
to  twelve  years  to  reach  the  floral  climax  in  which  it  expires, 
and  in  our  greenhouses  as  much  as  a  generation  or  two,  whence 
its  name  of  "  century  plant." 

In  contrast  to  such  cases,  precocious  reproductive  maturity 
occasionally  occurs.  We  have  already  referred  to  those 
dipterous  midges  {Ceddomyi^B),  in  which  the  larvae  for  succes- 
sive generations  become  reproductive,  though  only  partheno- 
genetically.  Very  striking  too  is  the  trematode  worm 
Gyrodactyhis^  which  recalls  the  mystical  views  of  the  prefor- 
mationists,  in  exhibiting  three  generations  of  embryos,  one 
within  the  other,  while  the  oldest  is  yet  unborn.  The  well- 
known  axolotl  of  Mexican  lakes,  though  with  its  persistent 
gills  in  a  sense  the  larval  form  of  Amblystoma,  attains  of  course 


244  THE    EVOLUTION    OF    SEX. 

to  sexual  maturity.  A  more  marked  precocity  has  been  ob- 
served in  the  x\lpine  salamander  {Trilon  alpestris).  In  higher 
organisms,  it  occasionally  happens  that  long  before  growth  has 
ceased  or  adolescence  been  reached  sexuality  sets  in,  especially 
in  the  male  sex,  but  this  is  fortunately  a  comparatively  rare 
pathological  occurrence.  In  one  set  of  organisms  precocious 
reproductive  maturity  has  been  of  paramount  importance,  viz., 
in  the  flowering  plants.  Here  the  prothallium  stage,  as  con- 
trasted wnth  the  vegetative,  has  been  much  reduced,  and  has 
remained  associated  with  or  been  absorbed  by  the  asexual 
generation.  This  is  to  be  in  part  explained  by  the  accelerated 
reproduction  of  the  prothallus,  comparable  to  a  similar  process 
which  has  reduced  the  separate  medusoid  sexual  persons  of  a 
hydroid  colony  to  mere  buds. 

§  2.  Menslniation. — The  process  of  menstruation  {menses,  catamenia), 
although  from  the  earliest  times  the  subject  of  medical  inquiry,  is  by  no 
means  yet  clearly  understood.  It  occurs  usually  at  intervals  of  a  lunar 
month  in  all  females  during  their  period  of  potential  fertility  (fecundity), 
and  so  far  from  being  confined  to  the  human  species,  has  been  observed  at 
the  period  of  "  heat  "  in  a  large  number  of  mammals.  Though  thus  clearly 
a  normal  physiological  process,  it  yet  evidently  lies  on  the  borders  of 
pathological  change,  as  is  evidenced  not  only  by  the  pain  which  so  fre- 
quently accompanies  it,  and  the  local  and  constitutional  disorders  which  so 
frequently  arise  in  this  connection,  but  by  the  general  systemic  disturbance 
and  local  histological  changes  of  which  the  discharge  is  merely  the  outward 
expression  and  result.  In  general  terms,  and  apart  from  ovulation, 
menstruation  may  be  described  as  a  periodic  discharge  of  blood,  glandular 
secretion,  and  cellular  detritus  from  the  lining  of  the  uteius.  After  from 
three  to  six  days  the  blood  ceases  to  appear,  and  the  lost  epithelium  is 
rapidly  replaced,  apparently  by  proliferation  from  the  necks  of  the  glands. 
By  the  ninth  or  tenth  day  the  mucous  coat  is  fully  healed,  and  the  begin- 
nings of  the  next  menstrual  process  recommence. 

The  age  at  which  the  process  commences  varies  with  race  and  climate, 
with  nutrition  and  growth,  with  habit  of  life  {e.g.,  with  difference  between 
town  and  country  life),  and  with  mental  and  moral  characteristics.  Of 
these,  however,  climate  seems  most  important  ;  thus,  while  in  Northern 
Europe  the  age  is  reckoned  at  the  beginning  of  the  fifteenth  year,  in  the 
tropics  it  commences  earlier,  in  the  ninth  or  tenth  year,  according  to 
some.  The  cessation  of  menstruation  usually  takes  place  between  the 
age  of  forty-five  and  fifty,  and,  somewhat  as  the  secondary  characteristics 
of  female  puberty  coincide  with  its  appearance,  a  less  distinct  reduction  of 
these  is  associated  with  its  close  ;  in  many  cases  secondary  resemblances 
to  the  masculine  type  may  supervene. 

The  old  theories  of  menstruation  were,  that  it  served  to  rid  the  system 
of  impure  blood,  that  it  simply  corresponded  to  the  period  of  "heat" 
observed  in  lower  animals,  or,  later,  that  it  was  associated  with  ovulation, 
— which  indeed  seems  broadly  to  correspond  with  the  end  of  the  menstrual 
period.     And  while  it  cannot  be  maintained  that  either  "heat  "  or  ovula- 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       245 

tion  are  necessarily  associated  with  menstruation  in  Homo,  there  can  be 
little  doubt  of  the  general  physiological  parallelism  of  all  three  processes. 
At  present  there  may  be  said  to  be  two  rival  theories.  According  to  the 
first  of  these,  the  process  is  viewed  as  a  kind  of  surgical  "  freshening"  of 
the  uterus  for  the  reception  of  the  ovum,  whereby  the  latter  during  the 
healing  process  can  be  attached  safely  to  the  uterine  wall.  The  other  view 
is  exactly  the  reverse  of  this.  Its  upholders  regard  the  growth  of  the 
mucous  coat  before  this  commencement  of  the  flow  as  a  preparation  for  the 
reception  of  an  ovum  if  duly  fertilised,  and  the  menstrual  process  itself  as 
the  expression  of  the  failure  of  these  preparations, — in  short,  as  a  consequence 
of  the  non-occurrence  of  pregnancy.  A  decided  majority  of  gynaecologists 
appear  to  incline  to  the  latter  view. 

The  process  may,  however,  be  expressed  in  more  general,  and  at  the 
same  time  more  fundamental  terms.  If  the  female  sex  be  indeed  pre- 
ponderatingly  anabolic,  we  should  expect  this  to  show  itself  in  distinctive 
functions.  Menstruation  is  one  of  these,  and  is  interpretable  as  a  means  of 
getting  rid  of  the  anabolic  surplus,  in  absence  of  its  consumption  by  the 
development  of  offspring, — ^just  as  it  is  intelligible  that  the  process  should 
stop  after  fertilisation,  when  replaced  by  the  demands  of  the  practically 
parasitic  foetus.  In  the  same  way,  the  occurrence  of  lactation,  after  this 
internal  parasitism  has  been  terminated  by  birth,  is  seen  to  be  reasonable. 
The  young  mammal  is  thus  enabled  to  become  what  is  practically  a 
temporary  ecto-parasite  upon  the  unfailing  maternal  anabolic  surplus  ;  and 
when  lactation  finally  ceases,  we  have  the  return  of  menstruation,  from 
which  the  whole  cycle  may  start  anew.  So  in  the  widely  different  yet 
deeply  similar  world  of  flowers,  the  distinctly  anabolic  overflow  of  nectar 
ceases  at  fertilisation,  and  the  surplus  of  continual  preponderant  anabolism 
is  drafted  into  the  growing  seed  or  fruit. 

§  3.  Sexual  Ujiion. — In  a  previous  chapter  we  have  noted 
the  passive  and  random  way  in  which  the  sex-elements  of  many 
of  the  lower  animals  are  liberated,  and  the  chance  manner  in 
which  they  are  brought  together  by  water-currents  and  the  like, 
though  this  may  not  be  quite  so  common  as  our  ignorance 
leads  us  to  suppose,  witness  the  recent  observation  of  the 
sexual  intertwining  of  Asterina  and  of  Aiitedon.  Yet  more 
in  plants  is  the  liberation  of  male  elements,  and  notably 
that  of  pollen -grains,  a  passive  dehiscence,  and  fertilisation 
a  matter  of  chance,  only  reduced  by  the  prodigal  wealth 
of  material.  Secure  as  the  methods  of  fertilisation  of  flowers 
by  the  aid  of  insects  often  are,  the  margin  of  risk  is  wide ; 
and  this  is  yet  more  marked  when  the  pollen  is  carried 
by  the  wind.  It  is  true  that,  both  in  plants  and  animals,  there 
are  subtle  attractions  between  the  essential  elements,  but  this 
is  only  at  a  close  range ;  and  the  external  union  is  in  many 
cases  none  the  less  random. 

It  must  be  allowed  that  the  primary  importance  of  the 
timely    encounter  of  the   ovum   and   spermatozoon    has    per- 


246 


THE    EVOLUTION    OF    SEX. 


petuated  in  the  various  groups  a  varied  series  of  adaptations 
securing  fecundation.  At  the  same  time,  the  increasing 
differentiation  of  the  sexes  has  in  the  higher  animals  been 
enhanced  by  psychical  as  well  as  physical  attractions,  thus 
more  and  more  ensuring  the  continuance  of  the  species. 


Male  of  Paper  Nautilus  (Argonauta),  with  its  modified 
arm.— From  Leunis. 

A  not  unfrequent  mode  of  fecundation  is  by  means  of  spermatophores, 
or  packets  of  spermatozoa.  These  may  be  seen  at  times  attached  to  the 
earth-worm,  or  found  within  the  leech  and  snail.  Even  in  newts  sperma- 
tophores are  formed,  which  are  taken  up  by  the  females. 

In  the  spider  the  spermatozoa  are  stored  in  a  special  receptacle  on  the 
palp,  and  hence  hastily  transferred  to  the  fierce  female.      In  cuttlefishes 


Diplozoon  paj-adoxuDi,  a  douljle  organism 
formed  from  the  union  of  two  distinct 
hermaphrodite  individual  trematodes 
(i9(/tf7;j!>a)  at  an  early  stage  in  their  life. 

this  mode  of  impregnation  is  yet  more  marked.  One  of  the  "arms"  of 
the  male,  much  modified  and  laden  with  spermatojihores,  is  thrust,  or  in 
many  cases  bodily  discharged  into  the  branchial  cavity  of  the  female,  where 
it  bursts.  Such  a  discharged  arm  was,  on  first  discovery,  regarded  as  a 
parasite,  and  hence  receivecl  the  name  of  Ilectocotylus.     A  curious  aberra- 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       247 

lion    from    the   ordinary  relations    is    figured   above,    where    two   distinct 
animals  [Diplozoon)  join  in  almost  life-long  union. 

In  many  cases  again,  especially  in  bony  fishes,  there  is  a  sexual  attrac- 
tion between  male  and  female,  but  without  any  copulation.  The  female, 
accompanied  by  'her  mate,  deposits  ova,  which  he  thereupon  fertilises 
with  spermatozoa.  A  slightly  more  advanced  stage  is  seen  in  the  frog. 
Fertilisation  is  still  outside  the  body  of  the  mother,  but  the  male,  embracing 
the  female,  liberates  spermatozoa  upon  the  eggs,  which  are  at  the  same  time 
laid. 

In  the  majority  of  cases,  however,  special  organs  for  emitting  and  for 
receiving  spermatozoa  are  developed,  and  copulation  occurs.  The  male 
organ  is  often  an  adaptation  of  some  structure  already  existing,  as  in  many 
crustaceans,  where  modified  appendages  form  external  canals  for  the  seminal 
fluid.  In  skates  and  other  gristly  fishes,  the  remarkably  complex  copula- 
tory  organs,  so-called  "claspers,"  are  in  close  connection  with  the  hind 
limb.  The  penis  of  higher  vertebrates  is  virtually  a  new  organ.  The 
copulation  may  be  quite  external,  as  in  crustaceans,  where  the  male  seizing 
the  female  deposits  spermatozoa  upon  the  already  laid  eggs.  Oftener,  how- 
ever, it  is  internal,  and  the  intromittent  organ  is  inserted  into  the  genital 
aperture  of  the  female.  True  copulation  may  occur  without  the  presence 
of  special  organs, — notably  in  the  case  of  many  birds,  where  the  cloaca  of  the 
male  is  apposed  to  that  of  the  female.  The  spermatozoa,  forcibly  expelled 
by  the  excited  male  organs,  pass  up  the  female  ducts,  probably,  in  part,  as 
the  result  of  peristalsis,  but  chiefly  at  least  by  their  own  locomotor  energy, 
and  one  of  them  may  eventually  fertilise  an  ovum.  In  addition  to  the 
intromittent  organ,  and  the  lower  portion  of  the  female  duct  which  receives 
it  during  copulation,  there  may  be  auxiliary  structures,  such  as  true 
claspers  for  retaining  hold  of  the  females.  The  limy  "cupid's  dart"  or 
"  spiculum  amoris  "  of  the  snail,  is  usually  interpreted  as  a  preliminary 
excitant. 

Three  further  notes  in  regard  to  higher  animals  are  requisite,  (i.)  There 
is  much  reason  to  believe  that  the  follicles  tend  to  burst  towards  the 
end  of  menstruation  ;  that  this  may  be  accelerated  by  copulation  ;  success- 
ful fertilisation  may  occur  at  any  period,  but  most  frequently  soon  after 
menstruation,  and  most  rarely  during  the  relatively  infertile  period  most 
distant  from  that  process.  (2.)  After  conception,  when  the  fertilised  egg 
has  begun  to  develop,  the  mouth  of  the  uterus  is  closed  by  a  secretion, 
which  prevents  the  entrance  of  other  spermatozoa  should  further  copula- 
tion occur.  (3.)  The  period  of  gestation,  i.e.,  between  the  fertilisation  of 
the  ovum  and  the  extrusion  of  the  fcetus,  varies  widely  in  mammals,  from 
about  18  days  in  opossum,  or  30  in  rabbit,  to  about  280  days  in  Homo  or 
600  in  the  elephant,  being  longer  in  the  more  highly  evolved  types.  But 
it  also  depends  on  size,  being  about  280  days  in  cow  and  150  in  sheep  ;  on 
number  of  offspring,  being  about  350  in  mare  and  60  in  dog  ;  and  on  the 
degree  of  maturity  at  birth,  being  420  in  giraffe  and  40  in  kangaroo. 

§  4.  Parturition. — In  many  cases,  e.g.,  marine  annelids, 
mature  ova  burst,  as  we  have  already  noted,  from  the  mother 
animal,  who  may  thenceforth  have  nothing  more  to  do  with 
them.  Liberation  of  ova  from  the  ovary  and  from  the  organism 
may  be  almost  coincident,  as  in  most  bony  fishes.       In  other 


248  THE    EVOLUTION    OF    SEX. 

cases,  the  ova  are  retained  within  the  mother  until  fertiUsed, 
but  are  expelled  not  long  after,  before  development  has  advanced 
to  any  marked  degree.  Such  eggs  are  often  furnished  with  the 
important  capital  of  nutriment,  so  familiar  in  the  case  of  birds, 
and  may  be  also  surrounded  by  chitinous,  horny,  membranous, 
or  limy  shells.  All  such  forms  of  birth  are  familiarly  described 
as  oviparous. 

In  numerous  invertebrates,  fishes,  amphibians,  and  reptiles, 
the  ova  develop  within  the  mother,  and  the  young  are  born 
more  or  less  actively  alive.  To  such  cases,  where  there  is  no 
nutritive  connection  between  parent  and  offspring,  the  term 
ovo-viviparous  used  to  be  applied.  They  were  contrasted  with 
oviparous  birth,  as  in  birds,  on  the  one  hand,  and  with  the 
viviparous  birth  of  mammals,  on  the  other.  It  is  the  well- 
known  characteristic  of  the  latter  that  there  is  an  intimate  nutri- 
tive connection  between  mother  and  offspring.  The  term  is  of 
little  use,  however,  for  the  cases  to  which  it  is  applied  shade  off 
towards  the  two  other  forms  of  birth.  Thus  among  gristly 
fishes  {Mustelus  Icevis  and  Carcharias),  in  the  curious  bony 
fish  Anabkps,  and  in  certain  lizards  {Trachydosauriis  and 
Cydodus),  a  somewhat  placenta-like  function  is  discharged  by 
the  yolk-sac  and  the  wall  of  the  oviduct ;  while  in  fishes, 
reptiles,  &c.,  oviparous  and  ovo-viviparous  birth  may  occur  in 
nearly  related  forms.  The  distinction  involved  in  the  term  is 
therefore  abandoned,  and  it  must  also  be  recognised  that  the 
difference  between  egg-laying  and  the  production  of  young 
actively  alive  is  only  one  of  degree.  Even  in  mammals,  which 
are  V\Y\\):ixous  par  excellence^  the  two  lowest  genera — the  duck- 
mole  and  the  echidna — are  oviparous.  The  common  grass- 
snake,  normally  oviparous,  has  been  induced,  in  artificial  condi- 
tions, to  bring  forth  its  young  alive,  and  this  is  probably  true  of 
other  forms.  The  parthenogenetic  generations  of  aphides  are 
usually  viviparous,  while  the  fertilised  eggs  are  laid  as  such. 

§  5.  Early  Nutrition. — The  early  nutrition  of  the  embryo, 
and  even  larva,  is  in  most  cases  an  absorption  of  the  legacy  of 
yolk  material,  which  is  probably  richest  in  the  eggs  of  birds. 
The  tadpole  of  the  frog  grows  and  exerts  itself  for  some  time 
before  it  begins  to  feed  at  the  expense  of  this  inheritance  of 
yolk.  Later  on,  in  the  frog  division  of  amphibians,  the  growth 
of  new  structures  appears  to  be  provided  for  by  the  nutritive 
absorption  of  the  tail,  the  larva  literally  living  upon  itself.  The 
same  is  true  in  the  elaborate  metamorphosis  of  echinoderm 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       249 

larvae.  In  many  cases,  the  cells  of  the  embryo,  independently 
and  actively,  devour  the  yolk  and  other  available  material, 
doing  so  after  the  amoeboid  fashion  technically  known  as 
intra-cellular.  At  the  same  time,  osmotic  currents  may  more 
passively  effect  the  like  result.  In  the  whelk  and  related  forms, 
a  curious  cannibalism  is  well  known  to  occur  among  the  crowd 
of  embryos  enclosed  within  a  common  capsule.  The  stronger 
and  older  devour  the  younger  and  weaker,— a  struggle  for 
existence  happily  of  exceptional  precociousness.  In  the 
higher  vertebrates  (above  amphibians),  foetal  membranes — 
amnion  and  allantois — are  developed,  in  addition  to  the  yolk- 
sac  which  encloses  the  yolk.  Of  these  the  amnion  is  mainly 
protective,  and  the  allantois  at  first  almost  wholly  respiratory. 
But  in  birds  (and  probably  to  a  slight  extent  in  reptiles)  the 
allantois  begins  to  assume  nutritive  functions,  assisting  in 
the  absorption  of  the  yolk.  In  placental  mammals,  however, 
a  nutritive  function  becomes  paramount,  the  allantois  forming 
the  greater  part  of  the  embryonic  side  of  the  placenta.  The 
yolk-sac  is  here  virtually  yolk-less,  but  in  lower  orders  may  absorb 
nutriment  as  it  did  in  birds,  though  from  a  different  source, — 
the  maternal  wall.  In  most  cases,  however,  what  was  incipient 
on  the  part  of  the  yolk-sac,  in  the  exceptional  elasmobranchs 
and  lizards  already  mentioned,  becomes  the  emphatic  function 
of  the  allantois, — namely,  the  establishment  of  a  vascular  or 
nutritive  connection  with  the  wall  of  the  maternal  uterus.  By 
this  means,  though  no  drop  of  blood  ever  passes  from  mother 
to  offspring,  a  very  intimate  osmotic  transfusion  is  effected. 

§  6.  Lactation. — If  menstruation  be  a  means  of  getting  rid 
of  anabolic  surplus,  in  absence  of  the  foetal  consumption,  lacta- 
tion is  still  more  an  anabolic  overflow,  adapted  to,  though  not 
of  course  originally  caused  by  the  offspring's  demands.  It  is 
at  the  same  time  evident  enough,  and  easily  verified  by  the 
histologist,  that  in  actual  occurrence  both  processes  are  kata- 
bolic,  involving  cellular  disruption  and  death.  That  peculiar 
liability  of  these  uterine  and  mammary  tissues  to  disease, 
which  furnishes  the  most  tragic  possibilities  of  the  life  of 
woman,  becomes  thus  less  mysterious.  We  can  understand  more 
readily  the  association  of  such  diseases  with  much  of  what  we 
are  pleased  to  generalise  as  civilisation,  and  view  more  hope- 
fully the  possibilities  of  their  enormous  diminution  by  the 
rational  hygiene  of  civilisation  properly  so-called. 

The  milk  or  mammary  organs  are  modified   skin-glands, 


250  THE    EVOLUTION    OF    SEX. 

probably  most  nearly  allied  to  the  ordinary  sebaceous  type, 
except  in  monotremes  which  appear  to  be  divergent.  Every 
one  knows  that  they  are  exclusive  characteristics  of  mammals, 
and  are  only  normally  functional  in  the  female  sex.  Rudi- 
mentary in  the  males,  they  may  even  there  produce  milk 
("witches'  milk")  at  birth,  puberty,  and  under  pathological 
conditions,  while  cases  have  been  put  on  record  of  men  who 
have  actually  given  suck.*  They  vary  greatly  in  position  and 
number,  a  large  number  being  doubtless  the  primitive  condi- 
tion. In  function,  after  the  birth  of  offspring,  the  surrounding 
tissue  is  specially  rich  in  white  blood-corpuscles,  which  probably 
form  some  of  the  structural  elements  of  the  milk.  It  has  also 
been  shown  that  the  nuclei  of  the  gland  cells  undergo  degene- 
ration, disruption,  and  expulsion,  and  that  they  in  all  likelihood 
form  the  casein  elements  of  the  nutritive  fluid. 

Before  birth,  the  mammalian  embryo  has  been  nourished 
through  the  placenta,  by  the  transfusion  already  referred  to. 
The  alimentary  canal  has  obviously  had  no  experience  in 
digestive  function.  Before  it  proceeds  to  digest  the  food  of  the 
parents,  it  is  put  through  a  course  of  what  Sollas  neatly  terms 
"gastric  education,''  by  feeding  upon  the  readily  assimilated 
mother's  milk. 

§  7.  Other  Secretions. — Every  one  has  heard  at  least  of 
"  pigeon's  milk,"  and  many  are  familiar  with  its  administration 
to  the  young  birds.  This  is  produced  by  both  sexes  for  a 
week  or  so  after  the  hatching  of  the  young,  and  is  the  result  of 
a  degeneration  of  the  cells  lining  the  crop.  Some  of  the  cells 
break  up,  others  are  discharged  bodily.  The  result  forms  a 
milky  emulsion-hke  fluid,  which  is  regurgitated  by  the  parents 
into  the  mouth  of  the  young  bird.  A  similar  substance  is  said 
to  occur  in  some  parrots. 

Of  some  interest  also  is  the  supra-salivation  which  occurs 
at  the  breeding  season  in  the  swiftlets  {Co/iocalia),  which  form 
the  edible  birds'  nests,  the  costly,  though  to  us  wofully  insipid, 
luxury  of  Chinese  epicures.  Certain  salivary  glands  become 
l)eculiarly  active  in  these  birds  when  breeding,  and  the  secre- 
tion, which,  according  to  Green,  consists  chiefly  of  a  substance 
akin  to  mucin,  is  used  to  form  the  snow-white  fibrous  nest. 

Take  only  one  other  instance  of  peculiar  secretion,  curiously 

*  Merriam  (Ilayden's  U.S.  Geol.  Survey,  VL,  p.  666)  gives  a  definite 
account  of  male  lactation  in  Leptis  bairdi. 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       25 1 


linked  to  the  above  by  one  of  those  profound  physiological 
unities  which  show  how  superficial  after  all  are  the  utmost 
contrasts  of  organic  form, — we  refer  to  the  viscid  threads  with 
which  the  male  stickleback  weaves  his  nest.  Mobius  has 
shown  that  the  kidneys  are  greatly  affected  by  the  mature 
testes ;  that  they  produce,  by  a  now^  normal  ]mthological  pro- 
cess, special  waste  or  katabolic  elements,  in  the  form  of  mucous 
threads.  The  male  gets  rid  of  this  uneasy  encumbrance  (which 
has  a  somewhat  parallel  pathological  equivalent  in  higher  ani- 
mals), by  rubbing  itself  against  objects,  and  thus  almost 
mechanically  has  been  evolved  the  familiar  weaving  of  the 
aquatic  nest. 


The  Nest  of  the  Stickleback  (Gastcrosteiis). — From  Thomas  Bolton. 

§  8.  Incubation. — The  physiological  sacrifice  of  the  female 
birds  does  not  end  with  providing  the  large  capital  of  nutritive 
material  with  which  the  germ  is  endowed,  but  is  continued  in 
all  the  patience  of  brooding.  In  passerine  birds  the  male 
relieves  the  female  in  her  task  of  love,  and  in  the  ostrich  tribe 
takes  the  duty  usually  upon  himself.  In  the  cuckoos  and  cow- 
birds  the  parental  care  is  shirked,  and  with  varying  degrees  of 
deliberateness  the  eggs  are  foisted  into  foster  nests,  and  the 
young  thus  put  out  to  nurse.  After  the  fatigue  of  reproduction 
it  is  perhaps  natural  enough  that  the  female  should  rest  awhile 
upon  the  eggs  in  the  shelter  of  the  nest,  and  since  there  is 
observed  to  be  an  increased  circulation  in  the  skin  of  the 


252 


THE    EVOLUTION    OF    SEX. 


abdominal  region  at  this  time,  it  has  been  argued  that  the  bird 
merely  sits  to  cool  itself!  This  view  has  been  supported  by  the 
cruel  experiment  of  singeing  off  the  feathers  from  the  same 
region  in  a  cock,  which  then  sat  to  cool  the  irritated  surface. 
Yet  the  increased  circulation  may  also  be  viewed  as  increased 
by  the  sitting ;  in  any  case,  the  patience  and  solicitude  of  the 
brooding,  and  the  subsequent  diligence  in  feeding  the  hatched 
young,  are  obviously  the  expression  of  genuine  parental  affec- 
tion. 


The  female  Surinam  Toad,  with  young  ones  on  its  hack.  —  From  Leunis. 


Here  too  one  must  include  the  retention  of  the  young  in 
skin  pouches,  exhibited  by  the  great  majority  of  marsupial 
mammals  and  by  the  echidna.  In  the  latter,  the  pouch  is  a 
simple  and  possibly  periodic  structure,  arising  from  an  insinking 
of  the  skin  in  the  mammary  region  of  the  abdomen.  Here  the 
eggs  are  somehow  or  other  stowed  away  and  the  young 
developed.  The  milk  glands  simi)ly  open  on  the  surface  of  the 
depression.     In  most  marsupials,  the  young,  which  are   born 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       253 

precociously  after  a  very  short  uterine  life,  are  sheltered  in 
similar,  but  more  developed,  pouches  of  the  skin,  within  which 
the  teats  open. 

In  oviparous  reptiles,  the  eggs  are  usually  left  to  hatch  of 
themselves,  aided  by  the  warmth  of  sun  and  soil.  "The 
female  python  disposes  herself  in  coils  round  her  eggs,  and 
incubates  them  for  a  prolonged  period,  during  which  the 
temperature  has  been  observed  to  rise  as  high  as  96°  F. 
within  the  coils." 

Some  exceedingly  curious  parental  adaptations  occur  among 
amphibians,  which  seem  to  have  made  numerous  experiments 


The  female  Nototre)na  niarsupiatuin, — an  amphibian,  with 
eggs  in  a  dorsal  sac,  which  is  shown  partly  uncovered. 
— F'rom  Cams  Sterne,  after  Giinther. 

on  the  matter.  Thus  in  the  Surinam  toad  {Pipa\  the  male 
spreads  the  ova  on  the  female's  back,  a  sort  of  erysipelas  sets 
in,  and  each  ovum  becomes  surrounded  by  a  skin-cavity  in  which 
the  tadpole  develops.  After  the  process  is  over,  the  skin  of  the 
back  is  renewed.  In  other  cases  this  mode  of  carrying  the 
ova  becomes  somewhat  more  definite ;  thus  in  Notodelphys  and 
Nototrema  the  eggs  are  stored  in  dorsal  pouches.  Nor  are  the 
males  without  their  share  in  the  task  of  parentage.  In  the 
obstetric  frog  {Alytes  obstetricajis)^  the  male  helps  to  remove 
the  eggs  from  the  female,  twists  them  in  strings  round  his  hind 
legs,  and  buries  himself  in  the  water  till  the  tadpoles  escape  and 


254  THE    EVOLUTION    OF    SEX. 

relieve  him  of  his  burden.  In  Rhinodenna  dai"wintt,  the  croak- 
ing sacs,  which  were  previously  used  for  amatory  calHng,  become 
enlarged  as  cradles  for  the  young. 


The  Sea-horse  {Hippocampus  suttulatiis). — From  the 
Atlas  of  the  Naples  Aquarium. 

Among  fishes,  parental  care  is  largely  in  abeyance,  and 
there  are  only  slight  hints  of  anything  in  the  way  of  incuba- 
tion.    In  a  siluroid  fish  {Aspredo),  the  female  deposits  her  ova 


The  female  of  the  "Paper  Nautilus    (^Argonauta  argd),  with  its 
brood -chamber. — After  Leunis. 

and  lies  upon  them  till  they  become  attached  to  the  spongy 
skin  of  the  belly,  very  much  as  happens  in  the  dorsal  attach- 
ment of  the  Surinam  toad.      After  hatching,  the  skin  excres- 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       255 

cence  is  smoothed  away.  In  Solenostoi?ia  (allied  to  pipe-fish)  the 
ventral  fins  unite  with  the  skin  to  form  a  pouch  in  w^hich  the 
eggs  are  retained.  In  other  cases,  it  is  the  male  which  incubates 
or  cares  for  the  ova.  Not  a  few  form  nests,  as  in  the  stickle- 
back, over  W'hich  they  keep  a  jealous  guard.  In  some  species 
of  Arins  the  eggs  are  carried  about  in  the  pharynx  ;  while  in 
the  sea-horses  a  pouch  is  developed  on  the  posterior  abdomen. 

Among  invertebrates,  brood-chambers  or  cradles  for  the 
young  are  not  uncommon.  The  capsules  of  hydroids,  the 
tent  of  spines  on  a  few  sea-urchins,  the  depressions  in  the  skin 
in  one  or  two  sea-cucumbers,  the  modified  tentacles  of  some 
marine  annelids,  the  dorsal  shell-chamber  in  water-fleas,  the 
incurved  abdomen  of  higher  crustaceans,  the  gill-cavities  of 
bivalves,  the  beautiful  brood-shell  of  the  argonaut,  illustrate  a 
habit  even  an  outline  of  which  is  beyond  our  limits. 

§  9.  Nefuesis  of  Reproductiofi. — ^Ve  have  already  shown 
how  reproduction  in  its  origin  is  linked  to  death.  The  primitive 
ruptures  by  which  the  protozoon  reduces  encumbering  bulk, 
saves  its  own  life,  and  multiplies  its  kind,  are  only  a  step  or 
two  from  more  diffuse  dissolution  which  is  death. 

The  association  of  death  and  reproduction  is  indeed  patent 
enough,  but  the  connection  is  in  popular  language  usually 
misstated.  Organisms,  one  hears,  have  to  die ;  they  must 
therefore  reproduce,  else  the  species  w^ould  come  to  an  end. 
But  such  emphasis  on  posterior  utilities  is  almost  always  only 
an  afterthought  of  our  invention.  The  true  statement,  as  far 
as  history  furnishes  an  answer,  is  not  that  animals  reproduce 
because  they  have  to  die,  but  that  they  die  because  they  have 
to  reproduce.  As  Goette  says,  "  it  is  not  death  that  makes 
reproduction  necessary,  but  reproduction  has  death  as  its 
inevitable  consequence."  This  of  course  refers  primarily  to 
the  incipient  forms  of  both  these  katabolic  processes. 

It  is  necessary  to  give  a  few  illustrations.  Goette  refers  to 
Hgeckel's  Magosphcera,  a  protozoon  which  just  as  it  had  formed 
for  itself  a  multicellular  body  broke  up  into  the  component  units. 
These  lived  on,  and  there  was  no  corpse,  but  at  the  same  time 
the  multicellular  colony  was  no  more.  Again  he  takes  the  case 
of  the  lowly  and  somewhat  enigmatical  orthonectids,  which 
Van  Beneden  has  classed  as  Mesozoa,  between  the  single- 
celled  and  the  stable  many-celled  animals.  Here  the  mature 
female  forms  numerous  germ-cells,  and  terminates  her  individual 
life  by  bursting.     The  germs  are  liberated,  the  mother  animal 


256 


THE    EVOLUTION    OF    SEX. 


has   been    sacrificed   in    reproduction.       "The    death    is    an 
altogether  inevitable  consequence  of  the  reproduction." 

Nor  is  this  sacrifice  confined  to  the  incipient  multicellular 
organisms.  Thus  in  some  species  of  the  annelid  Folygordius, 
the  mature  females  break  up  and  die  in  liberating  their  ova. 
This  is  approached,  but  suggestively  avoided,  in  a  genus  of 
capitellid  sea-worms  {Clitojiiastus).      The   whole  organism  is 


'11 


(I 


A  figure  of  cell  division  suggesting  the  internal  disruptions  and  re- 
arrangements of  the  nucleus  {a)  and  protoplasm. — From  Rauber. 

not  sacrificed,  but  only  an  abdominal  portion  of  the  body. 
This  is  in  fact  one  of  the  keynotes  to  reproductive  differentia- 
tion,— the  sacrifice  is  lessened,  and  the  fatality  thus  warded  off. 
But  again,  we  find  in  some  threadworms  or  nematodes  {e.g.^ 
Ascaris  dadyluris)  that  the  young  live  at  the  expense  of  the 
mother,  until  she  is  reduced  to  a  mere  husk.  In  fresh-water 
Polyzoa,  Kraepclin   notes  that  the  ciliated  embryo  leaves  the 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.        257 


maternal  body-cavity  through  a  prolapsus  uteri  of  the  sacrificed 
mother.  In  the  precocious  reproduction  of  some  midge  larvae 
{Ckironomus,  &c.),  the  production  of  young  is  fatal  through 
successive  generations. 

Both  Weismann  and  Goette,  though  with  different  interpreta- 
tions, note  how  many  insects  (locusts,  butterflies,  ephemerids, 
&c.)  die  a  few  hours  after  the  production  of  ova.  The 
exhaustion  is  fatal,  and  the  males  are  also  involved.  In  fact,  as 
we  should  expect  from  the  katabolic  temperament,  it  is  the 
males  which  are  especially  liable  to  exhaustion.     The  males  of 


Orthonectids,  showing  the  rupture  of  the  female  In   liberating 
the  germs. — From  Goette,  after  Julin. 

some  spiders  normally  die  after  fertilising  the  female,  a  fact 
perhaps  helping  to  throw  light  upon  the  sacrifice  of  others  to 
their  mates.  The  similarly  tiny  (ultra-katabolic)  male  rotifer — 
an  ideal  but  too  unpractical  lover,  with  not  even  an  alimentary 
canal — would  seem  usually  to  fail  and  expire  prematurely, 
leaving  the  female  to  undisturbed  parthenogenesis.  Every  one 
is  familiar  with  the  close  association  of  love  and  death  in  the 
common  mayflies.     Emergence  into  winged  liberty,  the  love- 

R 


258  THE    EVOLUTION    OF    SEX. 

dance  and  the  process  of  fertilisation,  the  deposition  of  eggs 
and  the  death  of  both  parents,  are  often  the  crowded  events  of  a 
few  hours.  In  higher  animals,  the  fatality  of  the  reproductive 
sacrifice  has  been  greatly  lessened,  }'et  death  may  tragically 
persist,  even  in  human  life,  as  the  direct  nemesis  of  love. 

The  temporarily  exhausting  effect  of  even  moderate  sexual 
indulgence  is  well  known,  as  well  as  the  increased  liability  to 
all  forms  of  disease  while  the  individual  energies  are  thus 
lowered. 

§10.  Ort^anic  Imuiortality.  —  Comparatively  little  is  yet 
know^n  about  the  length  of  life  among  lower  animals,  but  there 
is  no  reason  to  doubt  that  all  multicellular  organisms  die.  We 
have  just  emphasised  the  view  of  Goette  and  other  naturalists, 
that  reproduction  is  the  beginning  of  death;  which  is  not  incon- 
sistent with  the  apparent  paradox,  that  local  death  was  the 
beginning  of  reproduction.  Allowing,  then,  that  multicellular 
organisms  at  any  rate  are  mortal,  and  that  the  very  blossoming 
of  the  life  in  reproduction  is  fated  with  a  prophecy  of  death 
which  is  its  own  fulfilment,  we  have  to  face  two  questions, — 
What  of  death  in  the  Protozoa?  and,  In  what  sense  is  there  an 
immortality  throughout  the  organic  series? 

Often  enough  already,  in  the  preceding  pages,  we  have  had 
to  reiterate  the  contrasts  between  the  Protozoa  and  the  higher 
animals.  These  firstlings  are  physiologically  complete  in  them- 
selves, and  have  at  least  very  great,  if  not  unlimited,  powers  of 
self-recuperation.  They  leave  off  w^here  higher  animal  life 
begins,  that  is  to  say,  in  a  unicellular  state.  They  do  not  foim 
bodies.  Their  reproduction,  moreover,  is  in  the  majority 
simple  cell-division  into  two.  If  there  be  loss  of  individuality, 
there  is  hardly  loss  of  life.  Death  is  not  so  serious  when 
there  is  nothing  left  to  bury.  Nor  in  most  cases  can  one  half 
of  the  divided  unit  be  the  mother  individual,  and  the  other  the 
daughter,  for  the  two  appear  indistinguishably  the  same.  Thus 
an  idea,  broached  long  ago  by  Ehrenberg,  has  been  revived  and 
elaborated  by  several  naturalists,  and  especially  by  Weismann, 
that  the  Protozoa  are  virtually  immortal. 

In  Weismann's  own  words,  "  Natural  death  occurs  only 
among  multicellular  organisms,  the  single-celled  forms  escape 
it.  'I'here  is  no  end  to  their  development  which  can  be 
likened  to  death,  nor  is  the  rise  of  new^  individuals  associated 
with  the  death  of  the  old.  In  the  division  the  two  portions  are 
equal,  neither  is  the  older  nor  the  younger.     TIius  there  arises 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       259 

an  unending  series  of  individuals,  each  as  old  as  the  species 
itself,  each  with  the  power  of  living  on  indefinitely,  ever  divid- 
ing but  never  dying."  Ray  Lankester  puts  the  matter  tersely, 
"  It  results  from  the  constitution  of  the  protozoon  body  as  a 
single  cell,  and  its  method  of  multipHcation  by  fission,  that 
death  has  no  place  as  a  natural  recurrent  phenomena  among 
these  organisms." 

Some  limitations  must  be  noticed,  which  make  this  idea  of 
pristine  immortality  yet  more  emphatic.  It  is  only  asserted 
that  the  Protozoa  escape  "  natural  death,"  a  violent  fate  may  of 
course  await  them  like  any  other  organisms.  They  have  no 
charmed  life,  being  as  liable  to  be  devoured  as  those  of  higher 
degree.  In  relation  to  the  environment,  however,  their  sim- 
plicity gives  them  a  peculiar  power  of  avoiding  impending 
destiny.  The  habit  of  forming  protective  cysts  is  very  general, 
and  thus  enwrapped  they  can,  like  the  ova  and  a  few  of  the 
adults  of  some  higher  animals  (see  fig.  p.  193),  endure  desiccation 
with  successful  patience,  which  is  rewarded  by  a  rejuvenescence 
when  the  rain  revisits  the  pools.  But  the  doctrine  of  the 
"immortaHty  of  the  Protozoa"  refers  to  a  defiance  of  natural, 
not  violent,  death. 

The  psychological  objection  that  the  mother  psyche  is  really 
extinguished  when  she  divides  into  two,  intrudes  a  conception 
which  is  hardly  applicable.  The  individualities  are  doubled, 
nothing  is  really  lost.  Most  seriously  difficult  are  those  cases 
where  the  protozoon  produces  a  series  of  buds,  spores,  or 
division  units,  and  leaves  a  residual  core  or  unused  remnant 
behind  to  die.  But  in  regard  to  the  gregarines,  for  instance, 
where  such  a  remnant  is  left,  it  has  been  fairly  answered  that 
the  residue  is  rather  a  kind  of  excretion  than  the  parent  left  to 
perish  after  its  rejjroductive  sacrifice.  Weismann  is,  however, 
willing  to  admit  the  possibility,  that  in  the  suctorial  Acinetae, 
and  in  the  parasitic  gregarines,  which  are  both  somewhat 
removed  from  the  normal  protozoon  type,  there  may  be  cases 
of  true  mortality. 

Another  point  in  regard  to  which  experts  differ,  is  whether 
the  Protozoa  are  really  quite  self-recuperative.  l"hey  suffer 
injuries,  they  necessarily  waste,  portions  are  used  up  and  may 
be  ejected.  The  question  then  arises.  Are  those  acquired 
defects  obliterated,  or  do  they  become  intensified  ?  Is  the 
wasting  only  a  local  death,  or  is  it  the  beginning  of  a  true 
senescence  ?     This  is  a  question  which  can  only  be  answered 


26o  THE    EVOLUI'ION    OF    SEX. 

by  observation ;  a  priori  reasoning  is  here  futile.  The  most 
serious  criticism  of  Weismann's  view  is  due  to  Maupas. 
Already  we  have  noted  his  important  result,  that  conjugation  is 
essential  to  the  youth  of  the  species.  Without  this  incipient 
sexual  reproduction,  the  individuals  in  the  course  of  numerous 
successive  asexual  generations  grow  old.  The  nucleus  degen- 
erates, the  size  diminishes,  the  entire  energy  wanes,  the  senility 
ends  in  death.  Maupas  believes  that  all  organisms  are  fated  to 
suffer  decay  and  death,  and  protests  strongly  against  Weis- 
mann's theory  that  death  begins  with  the  Metazoa. 

It  must  be  noted,  however,  that  in  natural  conditions  the 
conjugation,  prohibited  in  Maupas's  experiments,  occurs  when 
it  is  wanted,  and  the  life  flows  on.  Furthermore,  conjugation 
has  not  been  shown  to  occur  in  many  Protozoa.  It  seems 
therefore  more  warrantable  to  insert  Maupas's  result  as  a  saving 
clause  to  Weismann's  doctrine,  than  to  regard  it  as  contra- 
dictory. The  conclusion  at  present  justifiable,  is  that  Protozoa 
not  too  highly  differentiated,  living  in  natural  conditions 
where  conjugation  is  possible,  have  a  freedom  from  natural 
death.  To  this  must  then  be  added  the  demonstrated  saving 
clause,  that  in  ciliated  infusorians,  conjugation,  which  here 
means  an  exchange  of  nuclear  elements,  is  the  necessary  con- 
dition of  eternal  youth  and  immortality. 

Accepting  then,  with  an  emphasised  proviso,  the  general 
conclusion  that  most,  if  not  all,  unicellular  organisms  enjoy 
immortality,  that  in  being  without  the  bondage  of  a  "  body  " 
they  are  necessarily  freed  from  death,  we  ])ass  to  consider  the 
second  question,  \Vhat  does  the  death  of  the  higher  and  multi- 
cellular organisms  really  involve? 

If  death  do  not  naturally  occur  in  the  Protozoa,  it  is  evident 
that  it  cannot  l)e  an  inherent  characteristic  of  living  matter. 
Vet  it  is  universal  among  the  multicellular  animals.  Death, 
we  may  thus  say,  is  the  price  i)aid  for  a  body,  the  penalty  its 
attainment  and  possession  sooner  or  later  incurs.  Now,  by  a 
body  is  meant  a  complex  colony  of  cells,  in  which  there  is 
more  or  less  division  of  labour,  where  the  component  units  are 
no  longer,  like  the  Protozoa,  in  possession  of  all  their  faculties, 
but  through  division  of  labour  have  only  restricted  functions 
and  limited  j)Ovvers  of  self-recujjeration.  Like  Maupas's  isolated 
family  of  infusorians,  the  cells  of  the  body  do  not  conjugate 
with  one  another  ;  and  though  they  divide  and  redivide  for  a 
season,  the  life  eventually  runs  itself  out. 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.       26 1 

A  moment's  consideration,  however,  will  show  that  in  most 
cases  the  organism  does  not  wholly  die.  Some  of  the  cells 
usually  escape  from  ihe  bondage  of  the  body  as  reproductive 
elements, — as,  in  fact.  Protozoa  once  more.  The  majority  of 
these  may  indeed  be  lost ;  eggs  which  do  not  meet  with  male 
elements  perish,  and  the  latter  have  even  less  power  of  inde- 
pendent vitality.  But  when  the  ova  are  fertilised,  and  proceed 
to  develop  into  other  individuals,  it  is  plain  that  the  parent 
organisms  have  not  wholly  died,  since  two  of  their  cells  have 
united  to  start  afresh  as  new  plants  or  animals.  In  other 
words,  what  is  new  in  the  multicellular  organism,  namely,  the 
"body,"  does  indeed  die,  but  the  reproductive  elements,  which 
correspond  to  the  Protozoa,  live  on. 

This  may  be  made  more  definite  in  the  following  diagram. 
There  it  is  seen  that  the  organism  starts  like  a  protozoon,  as  a 
single  cell,  or  usually  as  a  union  of  two  cells  in  the  fertilised 
ovum.    This  divides,  and  its  daughter-cells  divide  and  redivide. 


The  relation  between  reproductive  cells  and  the  body.  The  continuous  chain  of  dotted  cells  at 
first  represents  a  succession  of  Protozoa  ;  further  on,  it  represents  the  ova  from  which  the 
"bodies"  (undotted)  are  produced.  At  each  generation,  a  spermatozoon  fertilising  the 
liberated  ovum  is  also  indicated. 

They  arrange  themselves  in  layers,  and  are  gradually  ma])ped 
out  into  the  various  tissues  or  organs.  In  division  of  labour, 
they  become  restricted  in  their  functions,  and  specialised  in 
their  structure.  They  become  differentiated  as  muscle-cells, 
nerve-cells,  gland-cells,  and  so  on.  The  result  is  a  more  or 
less  complex  "body,"  unstable  in  its  equilibrium  because  of  its 
very  complexity,  composed  moreover  of  competing  cells  far 
removed  from  the  protozoon  all-roundness  of  function,  limited 
in  their  powers  of  recuperation,  and  emphatically  liable  to  local 
and  periodic,  or  to  general  and  final  death.  But  the  body  is 
not  all.  At  an  early  stage  in  some  cases,  sooner  or  later 
always,  reproductive  cells  are  set  apart.  These  remain  simple 
and  undifferentiated,  preserving  the  structural  and  functional 
traditions  of  the  original  germ-cell.  These  cells,  and  the  results 
of  their  division,  are  but  little  implicated  in  the  differentiation 


262  THE    EVOLUTION    OF    SEX. 

which  makes  the  multicellular  organism  what  it  is  ;  they  remain 
simple  primitive  cells  like  the  Protozoa,  and  in  a  sense  they 
too  share  the  protozoon  immortality.  The  diagram  shows  how 
one  of  these  cells,  separated  from  the  parent  organism  (and 
uniting  in  most  cases  with  a  germ-cell  of  different  origin) 
becomes  the  beginning  of  a  new  body,  and,  at  the  same  time, 
necessarily  the  origin  of  a  new  chain,  or  rather  of  a  continued 
chain  of  fresh  reproductive  cells. 

"  The  body  or  so?na,'"  Weismann  says,  "  thus  appears  to  a 
certain  extent  as  a  subsidiary  appendage  of  the  true  bearers  of 
the  life, — the  reproductive  cells."  Ray  Lankester  has  again 
well  expressed  this  : — -"Among  the  multicellular  animals,  certain 
cells  are  separated  from  the  rest  of  the  constituent  units  of  the 
body,  as  egg-cells  and  sperm-cells;  these  conjugate  and  continue 
to  live,  whilst  the  remaining  cells,  the  mere  carriers  as  it  were 
of  the  immortal  reproductive  cells,  die  and  disintegrate.  The 
bodies  of  the  higher  animals  which  die,  may  from  this  point  of 
view  be  regarded  as  something  temporary  and  non-essential, 
destined  merely  to  carry  for  a  time,  to  nurse,  and  to  nourish 
the  more  important  and  deathless  fission-products  of  the 
unicellular  egg." 

In  most  cases,  as  Weismann  insists,  it  is  more  correct  to 
speak  of  "  the  continuity  of  the  germinal  protoplasm  "  than  of 
the  continuity  of  the  germ-cells ;  but,  with  this  proviso,  the 
diagram  expresses  a  fact  most  important  in  understanding 
reproduction  and  heredity,  that  the  chain  of  life  is  in  a  real 
sense  continuous,  and  that  the  "  bodies "  which  die  are 
deciduous  growths,  which  arise  round  about  the  real  links. 
The  bodies  are  but  the  torches  which  burn  out,  while  the  living 
flame  has  passed  throughout  the  organic  series  unextinguished. 
The  bodies  are  the  leaves  which  fall  in  dying  from  the 
continuously  growing  branch.  Thus  although  death  take 
inexorable  grasp  of  the  individual,  the  continuance  of  the  life 
is  still  in  a  deep  sense  unaffected  ;  the  reproductive  elements 
have  already  claimed  their  protozoon  immortality,  are  already 
recreating  a  new  body ;  so  in  the  simplest  physical,  as  in  the 
highest  psychic  life,  we  may  say  that  love  is  stronger  than 
death. 


SPECIAL    PHYSIOLOGY    OF    SEX    AND    REPRODUCTION.        263 


SUMMARY. 

1.  Sexual  maturity  generally  occurs  towards  the  limit  of  growth,  is 
marked  by  liberation  of  reproductive  elements  and  by  secondary  charac- 
teristics, due  to  the  reaction  of  the  reproductive  function  on  the  general 
system.  Precocious  maturity  may  be  due  to  constitutional  or  environ- 
mental conditions,  and  has  been  of  much  importance  in  the  evolution  of 
flowering  plants. 

2.  Ivlenstruation  is  interpreted  as  a  means  of  getting  rid  of  the  anabolic 
surplus  of  the  female  in  absence  of  its  foetal  consumption. 

3.  Sexual  union,  at  first  very  passive  and  random,  becomes  active  and 
definite  with  the  gradual  evolution  of  sex  and  secondary  sexual  organs. 

4.  Birth  is  at  first  accomplished  by  rupture,  but  becomes  a  definite 
process  usually  effected  through  special  ducts.  Oviparous  and  viviparous 
birth  only  differ  in  degree. 

5.  Early  nutrition  is  usually  an  absorption  of  the  yolk,  but  in  mammals 
is  accomplished  by  osmotic  transfusion  from  the  blood  of  the  mother  to 
that  of  the  foetus. 

6.  Lactation  is  interpreted  as  an  anabolic  overflow. 

7.  Besides  milk,  there  are  other  secretions  associated  with  the  nutrition 
and  sheltering  of  the  young.  Pigeon's  milk,  edible  birds'  nests,  and  the 
mucous  threads  of  sticklebacks,  are  illustrations. 

8.  Incubation,  reaching  a  climax  in  birds,  is  paralleled  in  many  other 
classes. 

9.  Reproduction  and  death  both  represent  katabolic  crises.  Primitively, 
they  are  nearly  akin.  Reproduction  may  ward  off  death  from  the  Proto- 
zoon,  but  in  the  simplest  Metazoa  it  probably  caused  it. 

10.  The  Protozoa  come  nearer  immortality  than  other  organisms.  The 
fact  of  germinal  continuity  involves  an  organic  immortality. 


LITERATURE. 

For  the  special  physiology  of  sex  and  reproduction  consult  standard 
text-books  such  as  those  of  Foster,  Landois  and  Stirling,  and  especially 
Hensen's  work  already  often  cited. 

On  the  continuity  of  the  germ-plasma,  consult  recent  translation  of 
Weismann's  papers — "Heredity,"  Oxford,  1889;  while  a  full  biblio- 
graphy will  be  found  in  "  History  and  Theory  of  Heredity,"  by  J.  A. 
Thomson,  Proc.  Roy.  Soc.  Edin.,  1888  ;  and,  since  1886,  in  the  Zoological 
Record. 

On  the  nemesis  of  reproduction,  and  on  organic  immortality,  see  A. 
Goette,  "  Uber  den  Ursprung  des  Todes,"  Hamburg  and  Leipzig,  1883; 
and  A.  Weismann,  "  Ueber  die  Dauer  des  Lebens,"  Jena,  1882  ;  "  Ueber 
Leben  und  Tod,"  Jena,  1884;  E.  Maupas,  "Archives  de  Zoologie 
experimentale,"  1888. 


CHAPTER    XIX. 

Psychological  and  Ethical  Aspects. 

§  I.  Covunon  Groiaid  behveeti  Atiwia/s  and  Men. — Hitherto 
we  have  l)een  justifying  the  orthodoxy  of  an  anatomical  training, 
by  ahiiost  wholly  ignoring  the  fact  that  animals  have  a  psychic 
life,  or  only  mentioning  the  mere  neural  aspect  of  functions. 
Only  in  discussing  sexual  selection,  and  the  general  facts  of 
sexual  union  and  of  parentage,  have  we  intruded  words  like 
"  care,"  "  sacrifice,"  and  "  love."  A  purely  physiological  treat- 
ment of  sex  and  reproduction  is,  however,  obviously  incom- 
plete. It  would  be  rejected  with  scorn  in  reference  to  human 
life ;  it  must  be  equally  rejected  in  regard  to  the  higher 
animals,  which,  taken  together,  exhibit  the  analogues  of  almost 
every  human  emotion,  and  of  all  our  less  recondite  intellectual 
processes.  It  is  with  emotions  that  we  have  here  most  to  do  ; 
and  without  raising  the  difficult  question  whether  animals 
exhibit  any  emotions  exactly  analogous  to  those  which  in  man 
are  associated  with  the  "  moral  sense,"  "  religion,"  and  "  the  sub- 
lime," we  accept  the  conclusion  of  Darwin,  followed  by  Romanes 
and  others,  that  all  other  emotions  which  we  ourselves  experience, 
are  likewise  recognisable  in  less  perfect,  or  sometimes  more 
perfect,  expression  in  the  higher  animals.  Those  which  are 
associated  with  sex  and  reproduction  are  indeed  among  the 
most  patent ;  love  of  mates,  love  of  offspring,  lust,  jealousy, 
family  affection,  social  sympathies,  are  undeniable. 

§  2.  The  Love  of  Mates. — In  the  lowest  animals,  where  two 
exhausted  cells  flow  together  in  incipient  sexual  union,  there  is 
apparently  only  one  component  of  that  most  complex  musical 
chord  in  life  which  we  call  "  love."  There  is  physical  attraction, 
and  the  whole  process  is  very  much  a  satisfaction  of  proto- 
plasmic hunger. 

In  multicellular  animals,  the  liberation  of  sex-elements  is  at 
first  very  passive.  It  concerns  the  individual  alone.  Fertilisa- 
tion is  a  random  matter;  and  though  sex  exists,  sexual  attraction 
does  not. 


PSYCHOLOGICAL    AND    ETHICAL  ASPECTS.  265 

A  grade  higher,  true  sexual  union  begins  to  appear.  But 
at  first  this  simply  occurs  between  any  male  and  any  available 
female.  The  union  is  physiological,  not  psychological ;  there 
is  no  genuine  pairing,  and  it  would  be  folly  to  use  the  word 
love  in  such  cases. 

Gradually,  however,  for  instance  among  insects,  the  sexes 
associate  in  pairs.  There  is  some  psychic  sexual  attraction, 
often  accompanied  with  no  little  courtship,  but  much  more  im- 
portant is  the  occasional  maintenance  of  the  association  for  a 
lengthened  period.  There  may  even  be  co-operation  in  work, 
as  in  dung  rolling  beetles  such  as  Afeuc/ius,  where  the  two 
sexes  pursue  their  somewhat  disinterested  labours  together. 
The  male  and  female  of  another  lamellicorn  beetle  {Lcthrus 
cephalotes)  inhabit  the  same  cavity,  and  the  virtuous  matron  is 
said  greatly  to  resent  the  intrusion  of  another  male.  As  degene- 
rate offshoots  from  the  path  of  psychic  progress,  or  as  illustra- 
tions of  the  predominance  of  merely  physical  attraction,  one 
must  regard  such  prolonged  associations  of  the  two  sexes  as  are 
seen  in  the  formidable  parasitic  worm  Bilharzia^  where  the 
male  carries  the  female  about,  or  in  some  parasitic  crustaceans 
where  the  positions  are  reversed  (see  figs.  pp.  17  and  71). 

Among  the  cold-blooded  fishes,  the  battles  of  the  stickle- 
back with  his  rivals,  his  cai)tivating  manoeuvres  to  lead  the 
female  to  the  nest  which  he  has  built,  his  mad  dance  of 
passion  around  her,  and  his  subsequent  jealous  guarding  of  the 
nest,  have  often  been  observed  and  admired.  In  one  of  the 
sunfishes  the  male  and  female  alternate  in  guarding  the  ova. 
The  monogamous  habits  of  the  salmon,  and  the  frequently  fatal 
contests  between  rival  males  are  well  known.  Carbonnier  has 
beautifully  described  the  elaborateness  of  sexual  display  and  the 
ardency  of  passion  in  the  male  butterfly-fish,  and  also  in  the 
rainbow-fish  of  the  Ganges. 

The  amatory  croaking  of  frogs,  the  love-gambols  of  some 
newts,  the  curious  parental  care  of  some  male  amphibians 
mentioned  in  the  preceding  chapter,  and  the  like,  illustrate  the 
continuance  of  more  than  crude  physical  attraction  between  the 
sexes.  It  is  indeed  only  in  sexual  and  reproductive  relations 
that  the  amphibians  seem  to  wake  up  out  of  their  constitutional 
sluggishness. 

In  regard  to  reptiles,  little  is  known  beyond  the  exhibition 
of  sexual  passion  and  the  jealous  combats  of  rival  males.  Yet 
Romanes   refers   to   the  interesting  fact  that  when  a  cobra  is 


2  66  THE    EVOLUTION    OF    SEX. 

killed,  its  mate  is  often  found  on  the  same  spot  a  day  or  two 
afterwards. 

Among  birds  and  mammals,  the  greater  differentiation  of 
the  nervous  system  and  the  higher  pitch  of  the  whole  life  is 
associated  with  the  development  of  what  pedantry  alone  can 
refuse  to  call  love.  Not  only  is  there  often  partnershi}),  co- 
operation, and  evident  affection  beyond  the  limits  of  the 
breeding  periods,  but  there  are  abundant  illustrations  of  a  high 
standard  of  morality,  of  all  the  familiar  sexual  crimes  of  man- 
kind, and  of  every  shade  of  flirtation,  courtship,  jealousy,  and 
the  like.  There  is  no  doubt  that  in  the  two  highest  classes  of 
animals  at  least,  the  physical  sympathies  of  sexuality  have  been 
enhanced  by  the  emotional,  if  not  also  intellectual,  sympathies 
of  love.  Those  sceptical  on  this  point  should  consult  such  a 
work  as  liiichner's  "  Liebe  laid  Liebesleben  in  de?-  Thierwelt ^^'' 
which  contains  an  overflowing  wealth  of  instances. 

§  3.  Sexual  Attraction. — Mantegazza  has  written  a  work 
entitled  "The  Physiology  of  Love,"  in  which  he  expounds 
the  optimistic  doctrine  that  love  is  the  universal  dynamic ; 
and  from  this  Biichner  quotes  the  sentence,  that  "  the  whole 
of  nature  is  one  hymn  of  love."  If  the  last  word  be  used  very 
widely,  this  often  repeated  utterance  has  more  than  poetic 
significance.  But  even  in  the  most  literal  sense  there  is  much 
truth  in  it,  since  so  many  animals  are  at  one  in  the  common 
habit  of  serenading  their  mates.  The  chirping  of  insects,  the 
croaking  of  frogs,  the  calls  of  mammals,  the  song  of  birds, 
illustrate  both  the  bathos  and  glory  of  the  love-chorus.  The 
works  of  Uarwin  and  others  have  made  us  familiar  with  the 
numerous  ways,  both  gentle  and  violent,  in  which  mammals  woo 
one  another.  The  display  of  decorations  in  which  many  male 
birds  indulge,  the  amatory  dances  of  others,  the  love-lights  of 
glow-insects,  the  joyous  tournaments  or  furious  duels  of  rival 
suitors,  the  deliberate  choice  which  not  a  few  females  exhibit, 
and  the  like,  show  how  a  process,  at  first  crude  enough,  becomes 
enhanced  by  appeals  to  more  than  merely  sexual  appetite.  But 
it  is  hardly  necessary  now  to  argue  seriously  in  support  of  the 
thesis  that  love — in  the  sense  of  sexual  sympathy,  psychical  as 
well  as  physical —exists  among  animals  in  many  degrees  of 
evolution.  Our  comparative  psychology  too  has  been  too  much 
influenced  by  our  intellectual  superiority ;  but  while  this,  no 
doubt,  has  its  correspondingly  increased  possibilities  of  emo- 
tional   range,   it    does   not   necessarily  imply  a   corresponding 


PSYCHOLOGICAL    AND    f:THICAL    ASPECTS.  267 

emotional  intensity  ;  and  we  have  no  means  of  measuring,  much 
less  limiting,  that  glow  of  organic  emotion  which  so  manifestly 
flushes  the  organism  with  colour  and  floods  the  world  with 
song.  Who  knows  whether  the  song-bird  be  not  beside  the  man 
what  the  child-musician  is  to  the  ordinary  dulness  of  our  daily 
toil  and  thought?  The  fact  to  be  insisted  upon  is  this,  that 
the  vague  sexual  attraction  of  the  lowest  organisms  has  been 
evolved  into  a  definite  reproductive  impulse,  into  a  desire  often 
predominating  over  even  that  of  self-preservation ;  that  this 
again,  enhanced  by  more  and  more  subtle  psychical  additions, 
passes  by  a  gentle  gradient  into  the  love  of  the  highest  animals, 
and  of  the  average  human  individual. 

But  the  possibilities  of  evolution  are  not  ended,  and  though 
some  may  shrink  from  that  comparison  of  human  love  with  its 
analogues  in  the  organic  series,  the  theory  of  evolution  offers 
the  precise  compensation  such  natures  require.  Without  recog- 
nising the  possibilities  of  individual  and  of  racial  evolution,  we 
are  shut  up  to  the  conventional  view  that  the  poet  and  his  heroine 
alike  are  exceptional  creations,  hopelessly  beyond  the  everyday 
average  of  the  race.  Whereas,  admitting  the  theory  of  evolu- 
tion, we  are  not  only  entitled  to  the  hope,  but  logically  com- 
pelled to  the  assurance,  that  these  rare  fruits  of  an  apparently 
more  than  earthly  paradise  of  love,  which  only  the  forerunners  of 
the  race  have  been  privileged  to  gather,  or  it  may  be  to  see  from 
distant  heights,  are  yet  the  realities  of  a  daily  life  towards  which 
we  and  ours  may  journey. 

§  4.  Intellectual  and  Emotional  Differences  betiueen  the  Sexes. 
— We  have  seen  that  a  deep  difference  in  constitution  expresses 
itself  in  the  distinctions  between  male  and  female,  whether 
these  be  physical  or  mental.  The  differences  may  be  ex- 
aggerated or  lessened,  but  to  obliterate  them  it  would  be 
necessary  to  have  all  the  evolution  over  again  on  a  new  basis. 
What  was  decided  among  the  prehistoric  Protozoa  cannot  be 
annulled  by  Act  of  Parliament.  In  this  mere  outline  we  cannot 
of  course  do  more  than  indicate  the  relation  of  the  biological 
differences  between  the  sexes  to  the  resulting  psychological  and 
social  differentiations;  for  more  than  this  neither  space  nor 
powers  suffice.  We  must  insist  upon  the  biological  considera- 
tions underlying  the  relation  of  the  sexes,  which  have  been  too 
much  discussed  by  contemporary  writers  of  all  schools,  as  if 
the  known  facts  of  sex  did  not  exist  at  all,  or  almost  if  these 
were  a  mere  matter  of  muscular  strength  or  weight  of  brain. 


2  68  THE    EVOLUTION    OF    SEX. 

Even  a  recent  discussion,  which  is  professedly  from  the  bio- 
logical point  of  view,  that  of  Mr  Romanes,  sorely  disappoints 
us  in  this  regard. 

The  reader  need  not  be  reminded  of  the  oldest  and  most 
traditional  views  of  the  subjection  of  women  inherited  from  the 
ancient  European  order  ;  still  less  perhaps  of  the  attitude  of 
the  ordinary  politician,  who  supposes  that  the  matter  is  one 
essentially  to  be  settled  by  the  giving  or  withholding  of  the 
franchise.  The  exclusively  political  view  of  the  problem  has 
in  turn  been  to  a  large  extent  subordinated  to  that  cf  economic 
laissezfaire^  from  which  of  course  it  consistently  appeared  that 
all  things  would  be  settled  as  soon  as  women  were  sufficiently 
plunged  into  the  competitive  industrial  struggle  for  their  own 
daily  bread.  While,  as  the  complexly  ruinous  results  of  this 
inter-sexual  competition  for  subsistence  upon  both  sexes  and 
upon  family  life  have  begun  to  become  manifest,  the  more 
recent  economic  panacea  of  redistribution  of  wealth  has 
naturally  been  invoked,  and  we  have  merely  somehow  to 
raise  women's  wages. 

All  disputants  have  tolerably  agreed  in  neglecting  the 
historic,  nnd  still  more  the  biological  factors  ;  while,  so  far  as 
the  past  evolution  of  the  present  state  of  things  is  taken  into 
account  at  all,  the  ])osition  of  women  is  regarded  as  having 
simply  been  that  in  which  the  stronger  muscle  and  brain  of 
man  was  able  to  place  her.  The  past  of  the  race  is  thus  dei)icted 
in  the  most  sinister  colours,  and  the  whole  view  is  supposed  to 
be  confirmed  by  appeal  to  the  ])ractice  of  the  most  degenerate 
races,  and  this  again  as  described  with  the  scanty  sympathy  or 
imi)artiality  of  the  average  white  traveller,  missionary,  or  settler. 

As  we  have  already  said,  we  cannot  attempt  a  full  discussion 
of  the  question,  but  our  book  would  be  left,  as  biological  books 
for  the  most  ])art  are,  without  j^oint,  and  its  essential  thesis 
useless,  if  we  did  not,  in  conclusion,  seek  to  call  attention  to 
the  fundamental  facts  of  organic  difference,  say  rather  divergent 
lines  of  differentiation,  underlying  the  whole  problem  of  the 
sexes.  We  shall  only  suggest,  as  the  best  argument  for  the 
adoi)tion  of  our  standpoint,  the  way  in  which  it  becomes 
possible  relatively  to  affiliate  the  most  varied  standpoints.  We 
shall  not  so  readily  abuse  the  poor  savage,  who  lies  idle  in  the 
sun  for  days  after  his  return  from  the  hunting,  while  his  heavy- 
laden  wife  toils  and  moils  without  complaint  or  cease ;  but 
bearing  in  view  the  extreme  bursts  of  exertion  which  such  a 


PSYCHOLOGICAL    AND    ETHICAL    ASPECTS.  269 

life  of  incessant  struggle  with  nature  and  his  fellows  for  food 
and  for  life  involves  upon  him,  and  the  consequent  necessity 
of  correspondingly  utilising  every  opportunity  of  repose  to 
recruit  and  eke  out  the  short  and  precarious  life  so  indispen- 
sable to  wife  and  weans,  we  shall  see  that  this  crude  domestic 
economy  is  the  best,  the  most  moral,  and  the  most  kindly  attain- 
able under  the  circumstances.  Again,  the  traveller  from  town, 
who  thinks  the  agricultural  labourer  a  greedy  brute  for  eating 
the  morsel  of  bacon  and  leaving  his  wife  and  children  only 
the  bread,  does  not  see  that  by  acting  otherwise  the  total 
ration  would  soon  be  still  further  lowered,  by  diminished  earn- 
ings, loss  of  employment,  or  loss  of  health. 

I'he  actual  relations  of  fisherman  and  fishwife,  of  the  smallest 
farmer  and  his  wife,  seem  to  us  to  give  a  truer  as  well  as 
a  healthier  picture  of  antique  industrial  society,  than  those  we 
find  in  current  literature  ;  and  if  we  admit  that  such  life  is 
deficient  in  refinement  (although,  on  all  deeper  grounds,  from 
religion  to  ballad  poetry,  we  might  even  largely  dispute  this), 
it  has  still  nmch  to  teach  in  respect  of  simi)licity  and  health. 

The  old  view  of  the  subjection  of  women  was  not,  in  fact, 
so  much  of  tyranny  as  it  seemed,  but  roughly  tended  to  express 
the  average  division  of  labour;  of  course  hardships  were  fre- 
quent, but  these  have  been  exaggerated.  The  absolute  ratifi- 
cation of  this  by  law  and  religion  was  merely  of  a  piece  with 
the  whole  order  of  belief  and  practice,  in  which  men  crushed 
themselves  still  more  than  their  mates.  Being  absolute,  how- 
ever, such  theories  had  to  be  overthrown,  and  the  application 
of  the  idea  of  equality,  which  had  done  such  good  service  in 
demolishing  the  established  castes,  was  a  natural  and  serviceable 
one.  We  have  above  traced  the  development  of  this,  however, 
and  it  is  now  full  time  to  re-emphasise,  this  time  of  course 
with  all  scientific  relativity  instead  of  a  dogmatic  authority,  the 
biological  factors  of  the  case,  and  to  suggest  their  possible 
service  in  destroying  the  economic  fallacies  at  present  so  pre- 
valent, and  still  more  towards  reconstituting  that  complex  and 
sympathetic  co-operation  between  the  differentiated  sexes  in 
and  around  which  all  progress  past  or  future  must  depend. 
Instead  of  men  and  women  merely  labouring  to  produce  things 
as  the  past  economic  theories  insisted,  or  competing  over  the 
distribution  of  them,  as  we  at  present  think  so  important,  a 
further  swing  of  economic  theory  will  lead  us  round  upon  a 
higher  si)iral  to  the  direct  organic  facts.     So  it  is  not  for  the 


270  THE    EVOLUTION    OF    SEX. 

sake  of  production  or  distribution,  of  self-interest  or  mechanism, 
or  any  other  idol  of  the  economists,  that  the  male  organism 
organises  the  climax  of  his  life's  struggle  and  labour,  but  for  his 
mate ;  as  she,  and  then  he,  also  for  their  little  ones.  Pro- 
duction is  for  consumption  ;  the  species  is  its  own  highest,  its 
sole  essential  product.  The  social  order  will  clear  itself,  as  it 
comes  more  in  touch  with  biology. 

It  is  equally  certain  that  the  two  sexes  are  complementary 
and  mutually  dependent.  Virtually  asexual  organisms,  like 
Bacteria,  occupy  no  high  place  in  Nature's  roll  of  honour ; 
virtually  unisexual  organisms,  like  many  rotifers,  are  great 
rarities.  Parthenogenesis  may  be  an  organic  ideal,  but  it  is 
one  which  has  failed  to  realise  itself.  Males  and  females,  like 
the  sex-elements,  are  mutually  dependent,  and  that  not  merely 
because  they  are  males  and  females,  but  also  in  functions  not 
directly  associated  with  those  of  sex.  But  to  dispute  whether 
males  or  females  are  the  higher,  is  like  disputing  the  relative 
superiority  of  animals  or  plants.  Each  is  higher  in  its  own  way, 
and  the  two  are  complementary. 

While  there  are  broad  general  distinctions  between  the  in- 
tellectual, and  especially  the  emotional,  characteristics  of  males 
and  females  among  the  higher  animals,  these  not  unfrequently 
tend  to  become  mingled.  There  is,  however,  no  evidence  that 
they  might  be  gradually  obliterated.  The  sea-horse,  the  ob- 
stetric frog,  many  male  birds,  are  certainly  maternal ;  while  a  few 
females  fight  for  the  males,  and  are  stronger,  or  more  passionate 
than  their  mates.  But  these  are  rarities.  It  is  generally  true 
that  the  males  are  more  active,  energetic,  eager,  passionate,  and 
variable ;  the  females  more  passive,  conservative,  sluggish,  and 
stable.  The  males,  or,  to  return  to  the  terms  of  our  thesis,  the 
more  katabolic  organisms,  are  more  variable,  and  therefore,  as 
Brooks  has  especially  emphasised,  are  very  frequently  the 
leaders  in  evolutionary  })rogress,  while  the  more  anabolic  females 
tend  rather  to  i)reserve  the  constancy  and  integrity  of  the 
species ;  thus,  in  a  word,  the  general  heredity  is  perpetuated 
primarily  by  the  female,  while  variations  are  introduced  by  the 
male.  Yet  along  paths  where  the  reproductive  sacrifice  was  one 
of  the  determinants  of  progress,  we  shall  see  later  that  they 
must  have  the  credit  of  leading  the  way.  The  more  active 
males,  with  a  consequently  wider  range  of  experience,  may  have 
bigger  brains  and  more  intelligence  ;  but  the  females,  especially 
as  mothers,  have  indubitably  a  larger  and  more  habitual  share 


PSYCHOLOCxICAL    AND    ETHICAL    ASPECTS.  27 1 

of  the  altruistic  emotions.  The  males  being  usually  stronger, 
have  greater  independence  and  courage  ;  the  females  excel 
in  constancy  of  affection  and  in  sympathy.  The  spasmodic 
bursts  of  activity  characteristic  of  males  contrast  with  the 
continuous  patience  of  the  females,  which  we  take  to  be  an 
expression  of  constitutional  contrast,  and  by  no  means,  as  some 
would  have  us  believe,  a  mere  product  of  masculine  bullying. 
The  stronger  lust  and  passion  of  males  is  likewise  the  obverse 
of  predominant  katabolism. 

That  men  should  have  greater  cerebral  variability  and  there- 
fore more  originality,  while  women  have  greater  stability  and 
therefore  more  "  common  sense,"  are  facts  both  consistent  with 
the  general  theory  of  sex  and  verifiable  in  common  experience. 
The  woman,  conserving  the  effects  of  past  variations,  has  what 
may  be  called  the  greater  integrating  intelligence  ;  the  man,  in- 
troducing new  variations,  is  stronger  in  differentiation.  The 
feminine  passivity  is  expressed  in  greater  patience,  more  open- 
mindedness,  greater  appreciation  of  subtle  details,  and  con- 
sequently what  we  call  more  rapid  intuition.  The  masculine 
activity  lends  a  greater  power  of  maximum  effort,  of  scientific 
insight,  or  cerebral  experiment  with  impressions,  and  is 
associated  with  an  unobservant  or  impatient  disregard  of 
minute  details,  but  with  a  stronger  grasp  of  generalities.  Man 
thinks  more,  women  feels  more.  He  discovers  more,  but 
remembers  less  ;  she  is  more  receptive,  and  less  forgetful. 

§  5.  The  Love  for  Offspring. — Just  as  it  is  impossible  to 
point  to  the  stage  where  psychical  sympathies  enhance  the  re- 
productive impulse  into  the  love  of  mates,  so  we  cannot  tell 
where  parental  care  becomes  disinterested  enough  to  warrant 
our  calling  it  love  of  offspring.  For,  as  no  one  can  be  foolish 
enough  deliberately  to  ignore  the  sexual  or  physical  basis  of 
"love"  in  the  higher  and  highest  organisms,  so  it  must  be 
allowed  that  even  maternal  care  has  its  selfish  side.  To  take 
only  one  example,  that  of  lactation.  The  unrelieved  pressure 
in  the  mammary  glands  of  a  mother  animal  robbed  of  her  young 
is  no  doubt  largely  concerned  in  prompting  her  to  adopt  young 
ones  not  her  own,  yet  we  soon  see  these  established  in  her 
affections.  So  in  normal  cases,  there  naturally  remains  an  alloy 
which  prevents  us  from  regarding  even  maternal  care  as  alto- 
gether disinterested.  In  all  such  cases,  our  interpretations  risk 
an  undue  materialism  on  the  one  hand,  and  an  undue  transcend- 
entalism on  the  other ;   and  while  our    modern  temper   may 


272 


THE    EVOLUTION    OF    SEX. 


habitually  incline  us  to  the  former,  we  must  not  be  too  fond  of 
taking  for  granted  that  all  the  common  sense  is  on  that  side, 


A  Sea-cucumber,  or  Holothurlan  {Cucuninria  crocea),  with  numerous  young  attached 
to  the  skin. — From  Carus  Sterne,  after  "  Challenger"  Narrative. 


PSYCHOLOGICAL    AND    ETHICAL    ASPECTS. 


273 


for  we  must  remember  that  the  course  of  evolution  not  only  has 
been,  but  must  be,  towards  the  other. 

Among  animals  low  down  in  the  organic  series  there  often 
occurs,  as  we  have  already  noticed,  a  close  association  between 
mother  and  offspring.  Even  in  some  coelenterates,  worms,  and 
echinoderms,  the  offspring  cling  about  the  mother  animals,  and 
may    be   protected  in  various   kinds   of  brood-chambers.     In 


A  Male  "  Sea-spider,"  or  Pycnogonid,  carrying  the  ova. — 
After  Cams  Sterne, 

some  lowly  crustaceans,  the  young  may  return  to  the  shell- 
cavity  of  the  mother  after  hatching,  and  even  after  they  have 
undergone  a  moulting.  The  young  crayfish  are  said  to  return 
to  the  maternal  shelter  after  they  have  been  set  adrift.  The 
care  of  the  nurse-bees  for  their  charge,  though  not  exactly 
maternal,  deserves  to  be  recalled ;  and  the  way  in  which  ants 
save  the  cocoons  when  danger  threatens  is  well  known.     De 

s 


274 


THE    EVOLUTION    OF    SEX. 


Geer  describes  how  one  of  the  insects  infesting  plants  behaves 
to  her  young  brood  exactly  like  a  hen  with  her  chickens ;  and 
Bonnet  vividly  describes  a  case  where  a  mother  spider,  at  the 
mercy  of  an  ant-lion,  fought  for  her  eggs  at  the  sacrifice  of  her 
own  life.  Some  spiders,  too,  carry  their  young ;  and  some 
crustaceans,  like  Gammarus,  swim  along  with  their  young  ones, 
like  a  hen  among  her  chickens.  Some  cuttlefishes  take  pains 
in  keeping  their  egg  clusters  clean  and  safe  ;  while  even  the 
headless  fresh-water  mussel  retains  her  young,  when  there  is  no 
fish  present  to  which  they  may  attach  themselves.     In  fishes. 


Egg-Clusters  of  a  species  of  Cuttlefish.-  From  Von  Hayek. 

it  must  be  allowed  that  the  care,  if  at  all  evident,  is  usually 
paternal ;  in  amphibians,  it  is  rare ;  in  rei)tiles,  somewhat  more 
marked.  In  birds  and  mammals,  however,  parental  care  is 
general,  and  unquestionably  grows  into  love  for  offspring. 

§  6.  The  Habits  of  the  Cuckoo. — As  animals  exhibit  the 
analogues  of  the  human  virtues,  it  is  not  surprising  to  find 
the  occurrence  of  parallel  vices.  Those  of  much  magnitude, 
such  as  parental  negligence  or  cruelty,  are  however  rare,  for 
the  conditions  of  life  are  too  simple  to  admit  of  such  developed 


PSYCHOLOGICAL    AND    ETHICAL    ASPECTS.  275 

evils  as  in  human  society,  while  the  crimes  of  sexuality  are  also 
lessened  by  the  limitations  of  definite  breeding  seasons.  With- 
out exposing  the  details  of  the  crime  list,  it  will  be  instructive, 
as  a  concrete  illustration,  to  discuss  at  some  length  the  parasitic 
instinct  of  the  cuckoo. 

Every  schoolboy  knows  that  the  female  cuckoo  shirks  the 
brooding  sacrifice  usually  associated  with  bird  maternity. 
But  though  as  the  Scriptures  say,  somewhat  too  severely, 
of  the  ostrich,  "she  is  hardened  against  her  young  ones,  as 
though  they  were  not  hers,"  she  is  not  "deprived  of  wisdom  ;" 
by  an  elaborate  and  well-executed  trick  she  foists  her  several 
eggs,  at  intervals  of  a  few  days,  into  the  nests  of  various  birds, 
which  are  usually  insectivorous  and  suited  for  the  upbringing 
of  the  intruder.  The  foster-parents,  all  unconscious  of  being 
fooled,  hatch  the  cuckoo  egg  among  their  own.  The  nestling 
grows  rapidly,  and  is  a  dog  in  the  manger  by  birth.  Greedy 
and  jealous,  he  (the  pronoun  is  oftenest  correct)  soon  asserts 
his  monopoly  of  nest  and  food  and  care,  by  the  summary  evic- 
tion of  the  rightful  tenants,  whether  they  be  still  passive  in  ovo 
or  more  awkwardly  assertive  as  nestlings.  The  result  is  the 
success  of  the  stronger. 

Of  this  habit  there  are  various  explanations,  but  the  pre- 
valent one  regards  it  as  only  a  special  case  of  a  universal 
method  which  favours  selfishness.  Jenner  was  the  first  to 
emphasise  what  he  regarded  as  obvious  advantages  of  the 
trick.  The  bird  has  but  a  short  time  to  stay  in  its  breeding 
area,  and  much  to  do  in  that  short  time.  "Nature,"  he  said, 
"  has  a  call  upon  it  to  produce  a  numerous  progeny,"  and  as  it 
is  at  the  same  time  advantageous  to  migrate  early,  the  gain  of 
leaving  the  eggs  to  a  succession  of  other  birds  to  incubate  is 
manifest.  Darwin  supposed  the  habit  to  crop  u|)  as  a  mere 
fortuitous  variation,  as  it  occasionally  does  in  the  normally 
nesting  American  cuckoo.  The  result  was  an  advantage  to 
the  parent,  and  also  to  the  offspring  ;  the  former  got  away 
sooner,  the  latter  were  better  cared  for.  Those  that  learned 
the  trick  prospered,  those  that  did  not  were  eliminated  ;  and  so, 
in  virtue  of  its  natural  or  unnatural  success,  the  device  passed 
from  being  exceptional  to  become  universal,  became  in  fact  an 
inherited  specific  instinct.  Commenting  upon  this,  Romanes, 
in  a  surely  somewhat  sanguine  passage,  says  :  "  We  have  here 
a  sufficiently  probable  explanation  of  the  raison  d^etre  of  this 
curious  instinct ;  and  whether  it  is  the  true  reason,  or  the  only 


276  THE    EVOLUTION    OF    SEX. 

reason,  we  are  justified  in  setting  down  the  instinct  to  the 
creating  influence  of  natural  selection." 

But  against  the  supposition  that  a  mere  freak  has  been 
fostered  by  selection  into  a  habit,  it  must  be  noticed  that  the 
trick,  to  be  successful,  must  be  played  with  some  care.  It  is 
hardly  on  a  par  wMth  the  casual  use  made  by  a  partridge  of  a 
l)heasant's  nest,  or  by  a  gull  of  an  eider  duck's.  Again,  the 
advantages  to  the  parent,  apart  from  that  of  trouble  saved,  are 
somewhat  dubious.  Food,  Macgilivray  says,  remains  abundant, 
and  the  climate  which  does  not  injure  the  young  for  two 
months  longer  could  hardly  incommode  the  parents.  Nor  is 
the  case  improved  outside  the  British  area.  To  suppose,  on 
the  other  hand,  that  the  advantage  to  the  young  has  formed 
the  utilitarian  basis,  is  involved  in  difficulties.  We  cannot 
suppose  that  the  mother  bird  had  or  has  a  careful  forethought 
of  the  best  for  her  offspring  in  sending  them  out  to  nurse. 
Nor  is  it  easy  to  see  how  the  comfort  of  fostered  youth  will 
remain  as  an  impulse  to  the  adult  to  do  the  like  for  her  young 
in  turn.  The  difficulty  as  to  the  inheritance  of  such  a  freak, 
especially  with  the  preponderant  majority  of  males,  is  certainly 
appreciable.  The  common  difficulty  of  the  combination  of 
happy  circumstances  required  to  ensure  incipient  success  is 
unusually  great ;  the  young  bird  has  its  part  to  play  as  well  as 
the  parent ;  the  habit  is  not  generic,  yet  obtains  in  related 
genera,  and  also  in  the  widely  separated  starling-like  cow-birds. 

A  truer  view  of  the  habit  is  that  which  considers  it  as  a 
deliberate  expression  of  the  whole  constitution  of  the  bird. 

(i.)  The  general  character  of  the  cuckoo  is  very  significant. 
Brehm  describes  it  as  a  "discontented,  ill-conditioned,  pas- 
sionate, in  short  decidedly  unamiable  bird."  "  The  note  itself, 
and  the  manner  in  which  it  is  emitted,  are  typical  of  the  bird's 
habits  and  character.  The  same  abruptness,  insatiability, 
eagerness,  the  same  rage,  are  noticeable  in  its  whole  conduct." 
The  cuckoos  are  notoriously  unsociable,  even  in  migration 
individuahstic.  They  jealously  guard  their  territorial  "  pre- 
serves," and  verify  in  many  ways  the  old  myth  that  they  are 
si)arrow-hawks  in  disguise.  The  parasitic  habit  is  consonant 
with  their  general  character. 

(2.)  The  species  consists  predominantly  of  males.  The 
preponderance  is  probably  about  five  to  one,  though  one  observer 
makes  it  five  times  greater.  In  so  male  a  species,  it  is  not 
surprising  to  find  degenerate  maternal  instincts. 


PSYCHOLOGICAL    AND    ETHICAL    ASPECTS.  277 

(3.)  Reproduction  and  nutrition,  we  have  seen,  vary  in- 
versely. The  love-impulses  wane  before  those  of  hunger. 
Now  there  is  no  doubt  that  even  among  greedy  birds  the 
cuckoos  hold  a  very  high  rank.  They  are  remarkably  in- 
satiable, hungry,  gluttonous.  Even  the  anatomical  conditions 
asserted  by  some  to  be  important,  the  swollen  low-set  stomach, 
may  have  their  influence  in  the  cuckoo,  which  has  certain  other 
peculiarities,  though  the  same  conditions  may  be  overcome  in 
other  birds  which  remain  perfectly  natural.  It  might  almost  be 
suggested,  that  the  habit  of  feeding  so  largely  as  cuckoos  do  on 
hairy  caterpillars,  whose  indigestible  hairs  form  a  fretwork  in 
the  gizzard,  may  also  have  its  irritant,  gizzard-fretting,  dyspeptic 
influence.  But  the  main  point  is,  that  in  a  bird  with  so  strong 
nutritive  impulses,  it  is  little  wonder  the  reproductive  emotions 
are  degenerate.  There  is  too  much  hunger  and  gluttony  for 
the  higher  development  of  love. 

(4.)  The  reproductive  relations  of  the  sexes  are  at  a  lower 
level  than  polygamy,  or  rather  polyandry.  The  males  and 
females  do  not  pair  in  the  strict  sense,  there  is  no  keeping 
company,  though  the  males  are  said  to  be  passionate  during 
the  breeding  season.  Nor  is  the  female  in  its  adult  state 
externally  distinguishable  from  the  male. 

(5.)  The  reproductive  organs  of  both  sexes  are  very  small 
for  the  size  of  the  bird.  There  is  said  to  be  a  diminished 
blood  supply.  Little  wonder  then  that  the  reproductive  emo- 
tions are  in  degree  slightly  developed.  The  sluggish  parturition, 
at  intervals  of  six  to  eight  days,  is  also  striking  and  significant. 

(6.)  The  eggs  are  remarkably  small.  While  the  adult 
cuckoo  is  some  four  times  the  size  of  an  adult  skylark,  the 
eggs  are  about  the  same  size.  The  American  cuckoo,  which  is 
only  occasionally  parasitic,  lays  full-sized  eggs.  It  is  true  that 
the  size  of  an  egg  is  not  always  proportionate  to  the  size  of  the 
bird  ;  but  it  is  reasonable  to  believe,  that  when  a  bird  for  con- 
stitutional conditions  seems  to  require  all  it  can  for  itself,  then 
it  will  have  less  to  spare  for  its  re])roductive  sacrifice  To  say 
that  the  small  size  of  the  cuckoo's  egg  is  "an  adajjtation  in 
order  to  deceive  the  small  birds,"  seems  to  strain  the  natural 
selection  theory  to  the  breaking  point. 

(7.)  It  has  been  usual  in  discussing  beginnings  to  take 
some  cue  from  the  young  stages.  It  is  noteworthy,  in  this  light, 
to  emphasise  the  jealous  cruelty  of  the  young  foim,^ — a  fit  pro- 
phecy   of  the  adult    character.     In  the  restlessness  of  rapid 


278  THE    EVOLUTION    OF    SEX. 

growth,  the  nesthng  expresses  the  constitution  of  the  species  in 
its  selfish  monopolising  greed  and  insatiable  appetite.  Obser- 
vations are  recorded  of  the  persistence  of  the  cruel  disposition 
into  adolescence,  though  it  usually  wanes  with  the  anatomical 
peculiarity  of  the  back,  not  very  long  after  birth.  The  young 
form  at  any  rate  exhibits  the  essential  character  of  the  species. 

(8.)  Some  corroboration  is  obtamed  from  the  character  of 
the  American  cuckoo.  There  seems  no  doubt  that  it  is  occa- 
sionally parasitic,  and  it  is  interesting  to  note  that  observers 
speak  of  its  unnaturally  careless  indifference  for  the  fate  of  its 
young.  The  character  in  fact  is  less  markedly  evil ;  the  occa- 
sional parasitism  is  just  as  intelligible  as  the  occasional  "rever- 
sion "  of  our  cuckoo  to  ancestral  habits,  even  in  some  cases  to 
apparent  affection  for  the  young. 

(9.)  In  the  cow-birds,  again,  where  the  habit  occurs  in 
different  species  in  different  degrees  of  perfection  (if  the  term 
be  admissible),  the  character  is  strikingly  immoral.  In  one 
species  {Molothrus  cadius)^  a  nest  may  be  simply  stolen,  or  the 
rightful  nestlings  may  be  thrown  out,  or  actual  parasitism  may 
occur  as  an  exception.  In  M.  ca?ia?'iefisis,  the  eggs  may  be 
dropped  on  the  bare  ground,  or  fifteen  to  twenty  from  different 
parents  may  be  lazily  and  of  course  fatally  huddled  together  in 
one  nest.  Two  cuckoo  eggs  are  sometimes  found  in  one  nest. 
In  Af.  pecoris,  which  is  polygamous,  the  crime  has  been 
evolved,  and  the  habit  is  that  of  our  cuckoo,  one  egg  being 
laid  in  each  foster-nest.  The  important  point  is  the  general 
immorality  and  reproductive  carelessness,  which  in  one  species 
finds  expression  in  an  organised  device. 

Coiidiisioti. — The  general  character  of  the  birds — the  un- 
social life,  the  selfish  cruelty  of  the  nestlings,  and  the  lazy  para- 
sitic habit — have  a  common  basis  in  the  constitution.  The 
insatiable  appetite,  the  small  size  of  the  reproductive  organs, 
the  smallness  of  the  eggs,  the  sluggish  parturition,  the  rapid 
growth  of  the  young,  tiie  great  prei)onderance  of  males,  the 
al)sence  of  true  pairing,  the  degeneration  of  maternal  affection, 
are  all  correlated,  and  largely  explicable,  in  terms  of  tiie  funda- 
mental contrast  between  nutrition  and  re[)roduction,  between 
hunger  and  love.  Similar  unnatural  or  immoral  instincts  in 
other  birds,  in  mammals,  and  even  in  the  lower  animals,  are 
explicable  in  similar  terms.  The  cuckoo's  habit  is  a  natural 
outcrop  of  the  general  character  or  constitution,  only  one 
exi)ression  of  a  dominant  diathesis. 


PSYCHOLOGICAL    AND    ETHICAL    ASPECTS.  279 

In  his  recent  important  work  on  the  "  Origin  of  Species," 
Professor  Eimer  maintains  a  similar  view.  He  briefly  criticises 
the  Darwinian  explanation,  which  appears  to  him  to  postulate  too 
many  happy  combinations.  He  maintains  that  the  ancestral 
cuckoo  acted  deliberately  in  the  trick,  and  some  of  this  delibe- 
rateness  of  device  may  still  persist.  The  explanation  of  the 
unnatural  habit  is  to  be  found  in  the  bird's  whole  character  and 
mode  of  life.  In  this  connection  Eimer  emphasises  (a)  the 
vagabond,  restless  habit ;  (b)  the  looseness  of  the  sex  relations, 
strong  in  passion,  weak  in  love  ;  {c)  the  irregular  and  gluttonous 
nutrition  considered  in  relation  to  reproductive  stimulus ;  (d) 
the  slow  laying  of  the  eggs,  itself  dependent  upon  nutrition,  and 
pointing  to  physiological  conditions  which  modify  even  the 
deeply-rooted  impulse  and  instinct  to  brood ;  {e)  the  degenera- 
tion of  social  instincts,  and  the  preponderance  of  the  egoistic. 

^  7.  Egoism  and  Altrnisni. — I'he  optimism  which  finds  in 
animal  life  only  "  one  hymn  of  love  "  is  inaccurate,  like  the 
pessimism  which  sees  throughout  nothing  but  selfishness.  Littre, 
Leconte,  and  some  others  less  definitely,  have  more  reasonably 
recognised  the  co-existence  of  twin  streams  of  egoism  and 
altruism,  which  often  merge  for  a  space  without  losing  their 
distinctness,  and  are  traceable  to  a  common  origin  in  the 
simplest  forms  of  life.  In  the  hunger  and  reproductive  attrac- 
tions of  the  lowest  organisms,  the  self-regarding  and  other- 
regarding  activities  of  the  higher  find  their  starting-point. 
Though  some  vague  consciousness  is  perhaps  co-existent  with 
life  itself,  we  can  only  speak  with  confidence  of  psychical 
egoism  and  altruism  after  a  central  nervous  system  has  been 
definitely  established.  At  the  same  time,  the  activities  of  even 
the  lowest  organisms  are  often  distinctly  referable  to  either 
category. 

A  simple  organism,  which  merely  feeds  and  grows,  and 
liberates  superfluous  portions  of  its  substance  to  start  new  exist- 
ences, is  plainly  living  an  egoistic  and  individualistic  life.  But 
whenever  we  find  the  occurrence  of  close  association  with  another 
form,  we  find  the  first  rude  hints  of  love.  It  may  still  be  almost 
wholly  an  organic  hunger  which  prompts  the  union,  but  it  is 
the  beginning  of  life  not  wholly  individualistic.  Hardly  dis- 
tinguishable at  the  outset,  the  primitive  hunger  and  love  become 
the  starting-points  of  divergent  lines  of  egoistic  and  altruistic 
emotion  and  activity. 

The   differentiation  of  separate  sexes ;    the  production  of 


28o 


THE    EVOLUTION    OF    SEX. 


offspring  which  remain  associated  with  the  parents ;  the  occur- 
rence of  genuine  pairing  beyond  the  Hmits  of  the  sexual  period  : 
the  estabHshment  of  distinct  families,  with  unmistakable  affec- 
tion between  parents,  offspring,  and  relatives;  and  lasdy,  the 
occurrence  of  animal  varieties  wider  than  the  family, — mark 
important  steps  in  the  evolution  of  both  egoism  and  altruism. 

Ideal  unity. 


society, 


family. 


offspring. 


mates. 


N    V   R 


Protoplasmic  identity. 
Diagrammatic  Representation  of  the  Relations  between  Nutritive, 
Self-Maintaining,     or    Egoistic,    and     Reproductive,    Species- 
Regarding,  or  Altruistic  Activities. 

The  diagram  sums  up  the  important  facts.     There  are  two 
divergent  lines  of  emotional  and  practical  activity, — hunger, 


PSYCHOLOGICAL    AND    ETHICAL    ASPECTS.  251 

self-regarding,  egoism,  on  the  one  hand ;  love,  other-regarding, 
altruism,  on  the  other.  These  find  a  basal  unity  in  the  primi- 
tively close  association  between  hunger  and  love,  between 
nutritive  and  reproductive  needs.  Each  plane  of  ascent  marks 
a  widening  and  ennobling  of  the  activities ;  but  each  has  its 
corresponding  bathos,  when  either  side  unduly  preponderates 
over  the  other.  The  actual  path  of  progress  is  represented  by 
action  and  reaction  between  the  two  complementary  functions, 
the  minghng  becoming  more  and  more  intricate.  Sexual  attrac- 
tion ceases  to  be  wholly  selfish ;  hunger  may  be  overcome  by  love ; 
love  of  mates  is  enhanced  by  love  for  offspring ;  love  for  off- 
spring broadens  out  into  love  of  kind.  Finally,  the  ideal  before 
us  is  a  more  harmonious  blending  of  the  two  streams. 


262  THE    EVOLUTION    OF    SEX. 


SUMMARY. 

1.  In  most  of  the  emotions,  and  in  the  simpler  intellectual  processes, 
there  is  common  ground  between  animals  and  men.  This  is  especially  true 
of  the  emotions  associated  with  sex  and  reproduction. 

2.  The  love  of  mates  has  its  roots  in  physical  sexual  attraction,  but  has 
been  gradually  enhanced  by  psychical  sympathies. 

3.  The  means  of  sexual  attraction  rise  from  the  crude  and  physical  to 
the  subtle  and  psychical,  wiih  the  growth  of  love. 

4.  The  intellectual  and  emotional  differences  between  the  sexes  are 
correlated  with  the  deep-seated  constitutional  differences.  Males  and 
females  are  complementary,  each  higher  in  its  own  way. 

5.  The  love  for  offspring  has  grown  as  gradually  as  the  love  for  mates. 
Even  lactation  and  maternal  care  may  be  in  part  egoistic.  Except  in  a 
few  precociously  tender  animals,  genuine  love  for  offspring  is  only  emphatic 
in  birds  and  mammals,  where  the  reproductive  sacrifice  of  the  mother  has 
also  been  increased. 

6.  The  cuckoo  illustrates  the  evolution  of  a  criminal  habit,  mainly  due 
to  constitutional  conditions. 

7.  Egoism  and  altruism  have  their  roots  in  the  primary  hunger  and 
love,  or  nutritive  and  reproductive  activities.  The  divergent  streams  of 
emotion  and  activity  have  a  common  origin,  subtly  mingle  at  various  turn- 
ing-points, and  ought  to  blend  more  and  more  in  one. 


LITERATURE. 

See  works  on  Sexual  Selection  cited  at  Chap.  I. 

EiMER,   G.  H.   T. — Die  Entstehung  der  Arten  auf  Grund  von  Vererben 

Erworbener  Eigenschaften  nach  den  Gesetzen  Organischen  Wachsens. 

Jena,  1888. 
BiJCHNER,  L. — Liebe  und  Liebeslel)en  in  der  Thierwelt.    Berlin,  1879. 
Roi.FH,  W.  Yi.  —  Op.  cit. 
Romanes,  G.  J. — Animal  Intelligence.      Internat.   Sci.   Series.       Fourth 

edition,  1886;  and  Mental  Evolution  in  Animals,  by  the  same. 
Thomson,  J.  A.— A  Theory  of  the  Parasitic  Habit  of  the  Cuckoo.      Proc. 

Roy.  Phys.  Soc.    Edin.  1888. 
Sec  also  Carus   Sterne's    most    admirable    of    general   natural    history 

books — Werden  und  Vergehen.     Third  edition.      Berlin,  1886. 
Ploss. — Das    Weib    in    der    Natur    und    Volkerkunde.     Second    edition. 

Leipzig,  1887. 
Mantegazza,  p. — Die  Physiologic  der  Liebe;    Die  Hygiene  der  Liebe  ; 

Anthropologisch-Kulturhistorische   Studien    iiber   die   Geschlechtsver- 

haltnisse  des  Menschen.     Jena. 


CHAPTER  XX. 

Laws  of  Multiplication. 

§  I.  Rate  of  Reproduction  and  Rate  of  Increase. — \\'e  know 
much  more  about  the  rate  at  which  organisms  reproduce,  than 
about  the  rate  at  which  the  number  of  adults  in  reahty  increases 
or  decreases.  The  one  fact  may  be  ascertained  by  observation  ; 
the  other  involves  comparative  statistics,  which  are  difficult 
enough  to  obtain,  even  for  the  human  species.  The  rate  of 
reproduction  depends  upon  the  constitution  of  the  individual 
and  its  immediate  environment,  including,  above  all,  its  nutri- 
tion. The  rate  of  increase  or  decrease  depends  ui)on  the  wide 
and  complex  conditions  of  the  entire  animate  and  inanimate 
environment,  or  upon  the  degree  of  success  in  the  struggle  for 
existence. 

That  there  are  enormous  differences  in  the  rates  of  repro- 
duction is  very  evident.  Maupas  tells  us  how  a  single  infu- 
sorian  becomes  in  a  week  the  ancestor  of  a  progeny  only 
computable  in  millions, — of  numbers  which  the  progeny  of  a 
pair  of  elephants,  supposing  they  all  lived  their  natural  term  of 
years,  would  not  attain  to  in  five  centuries.  Again,  Huxley 
calculates  that  the  progeny  of  a  single  parthenogenetic  plant- 
louse — supposed  again  to  live  a  charmed  life — would  in  a  few 
months  literally  outweigh  the  population  of  China.  The  geo- 
metrical ratio  of  reproduction,  so  often  emphasised,  would 
indeed  have  startling  results  if  it  involved  real,  and  not  merely 
potential,  increase. 

That  it  does  sometimes  realise  itself  for  short  periods  or 
special  areas  of  favourable  conditions  is  well  known  ;  for  in- 
stance, in  the  periodic  plagues  of  insects,  or  in  the  still  unmas- 
tered  rabbit  pest  of  Australia.  But  in  the  established  fauna 
and  flora  of  a  country,  without  intruded  importations  or  marked 
climatic  changes,  the  rise  and  fall  of  population  is  seldom 
emphatic.     The  rate  of  reproduction  is  only  one  factor  in  the 


284  THE    EVOLUTION    OF    SEX. 

numerical  strength  of  the  species  or  in  its  increase.  The 
common  tapeworm  produces  myriads  of  embryos,  but  these 
have  only  one  chance  in  eighty-five  millions  (it  is  said)  of 
succeeding.  Many  common  and  numerous  animals  repro- 
duce very  slowly.  That  some  species  are  on  the  increase,  e.g.^ 
bacteria,  under  the  unprecedentedly  favourable  conditions  which 
our  recent  "  industrial  progress "  affords,  while  other  species 
are  on  the  decrease,  e.g.^  many  birds,  is  certain  ;  but  the  rate 
of  reproduction  is  not  a  direct  condition  in  either  case. 

I5  2.  History  of  Discussio7i  on  Rate  of  Reproduction. — In  this, 
as  in  not  a  few  other  rases,  the  biologist  is  profoundly  indebted 
to  the  student  of  social  questions,  for  no  adequate  attention  was 
paid  to  the  laws  of  multiplication  before  the  appearance  of  the 
epoch-making  "theory  of  population"  of  Malthus,  nor  is  it  yet 
possible  or  ])rofitable  to  isolate  the  human  question  from  the 
general  one.  Malthus's  fundamental  proposition  is  indeed 
usually  softened  from  its  earliest  form — that  ])opulation  tends 
to  increase  in  geometrical,  subsistence  only  in  arithmetical 
ratio — into  the  simple  statement  that  population  tends  to  out- 
run subsistence,  but  has  none  the  less  served  as  a  base  of 
weighty  deductions  for  both  the  naturalist  and  the  economist. 
From  Darwin's  standpoint,  the  "  positive  checks  "  to  population 
(disease,  starvation,  war,  infanticide),  and  the  "prudential" 
(moral  or  birth-restricting)  checks,  come  to  be  viewed  as  special 
forms  of  natural  or  artificial  selection,  while  the  fundamental 
induction  has  been  extended  throughout  nature  as  the  essential 
condition  of  the  struggle  for  existence.  After  long  dispute,  the 
induction  of  Malthus  gained  acceptance,  followed  by  wide 
deductive  use  and  abuse,  among  economists.  Yet,  fundament- 
ally important  as  the  subject  thus  is  to  naturalist  and  economist 
alike,  the  former  has  not  as  yet  effected  any  thorough  investi- 
gation of  the  conditions  of  multiplication,  or  even  usually 
incorporated  the  keen  analysis  which  we  owe  to  Spencer,  while 
the  economic  theorist  or  disputant  frequently  still  emi)loys  the 
doctrine  even  in  its  jjre-Darwinian  form.  It  is  thus  doubly 
needful  to  summarise,  as  briefly  as  may  be,  Spencer's  elaborate 
statement  of  the  laws  of  multijjlication. 

§  3.  Siuniuary  of  Spence7-''s  Analysis. — Different  species  exhil)it  different 
degrees  of  fertility,  which  have  l)econie  established  in  process  of  evolution 
like  the  organisms  themselves.  To  understand  this  ])articu]ar  adaptation 
of  function  to  conditions  of  existence,  of  organism  to  environment,  we  may 
analyse  these  into  their  respective  factors.  It  is  evident  that  in  the  environ- 
ment of  any  species  there  are  many  conditions  with  which  its  indi\idua]s 


LAWS    OF    MULTIPLICATION.  285 

establish  a  moving  equilibrium,  sooner  or  later  overthrowri  in  death.  To 
prevent  extinction,  the  organism  meets  these  environing  actions  in  two 
distinct  ways, — (i)  by  individual  adaptations,  active  thrusts  or  passive 
parries  ;  (2)  by  the  production  of  new  individuals  to  replace  those  over- 
thrown,— in  other  words,  by  genesis.  The  latter  may  occur,  as  we  have 
seen,  in  varied  forms,  sexual  or  asexual,  and  at  various  rates,  which  depend 
upon  age,  frequency,  fertility,  and  duration  of  reproduction,  together  with 
amount  and  nature  of  parental  aid.  These  actions  and  reactions  of  environ- 
ment and  organism  admit  of  another  grouping  in  more  familiar  terms,  into 
two  conflicting  sets, — {a)  the  forces  destructive  of  race  ;  (/?)  the  forces  pre- 
servative of  race. 

Leaving  aside  cases  in  which  permanent  predominance  of  destructive 
forces  causes  extinction,  and  also,  as  infinitely  improbable,  cases  of  perfectly 
stationary  numbers,  the  inquiry  is  : — In  races  that  continue  to  exist,  what 
laws  of  numerical  variation  result  from  these  variable  conflicting  forces 
that  are  respectively  destructive  or  preservative  of  race  ?  How  is  the 
alternate  excess  of  one  or  other  rectified  ?  A  self-sustaining  balance  must 
exist ;  the  alternate  predominance  of  each  force  must  initiate  a  compensa- 
tory excess  of  the  other  ;  how  is  this  to  be  explained  ? 

When  favourable  circumstances  cause  any  species  to  become  unusually 
numerous,  an  immediate  increase  of  destructive  influences,  passive  as  well 
as  active,  takes  place  ;  competiticui  becomes  keener  and  enemies  more 
abundant,  and  conversely.  Yet  this  is  not  the  sole,  much  less  the  perma- 
nent, means  of  establishing  a  balance  ;  nor  does  it  explain  either  the 
differences  in  the  rate  of  fertility  and  mortality,  or  the  adaptation  of  one  to 
the  other.     This  minor  adjustment  in  fact  implies  a  major  one. 

The  forces  preservative  of  race  were  seen  above  to  be  two, — power  to 
maintain  individual  life,  and  power  to  generate  the  species.  Now,  in  a 
species  which  survives,  given  the  forces  destructive  of  race  as  a  constant 
quantity,  those  preservative  of  race  must  be  a  constant  quantity  also  ;  and, 
since  the  latter  are  two,  the  individual  plus  the  reproductive,  these  must 
vary  inversely,  one  must  decrease  as  the  other  increases.  To  this  law 
every  species  must  conform,  or  cease  to  exist.  Let  us  restate  this  at  greater 
length.  A  species  in  which  .self-preservative  life  is  low,  and  in  which  the 
individuals  are  accordingly  rapidly  overthrown  in  the  struggle  with  the 
destructive  forces,  must  become  extinct,  unless  the  other  race-preservative 
factor  be  proportionally  strengthened, — unless,  that  is  to  say,  its  reproductive 
power  become  proportionally  great.  On  the  other  hand,  if  both  preserva- 
tive factors  be  increased,  if  a  species  of  high  self-preservative  power  were 
also  endowed  with  powers  of  multiplication  beyond  what  is  needful,  such 
success  of  fertility,  if  extreme,  would  cause  sudden  extinction  of  the  species 
by  starvation,  and  if  less  extreme,  and  so  effecting  a  permanent  increase  of 
the  numbers  of  the  species,  would  next  bring  about  such  intenser  competi- 
tion, such  increased  dangers  to  individual  life,  that  the  great  self-preserva- 
tive power  would  not  be  more  than  sufficient  to  cope  with  them. 

In  short,  then,  we  have  reached  the  a  priori  principle,  that  in  races 
which  continuously  survive,  in  which  the  destructive  forces  are  balanced 
by  the  preservative  ones,  there  must  be  an  inverse  proportion  between  the 
power  10  sustain  individual  life  and  the  power  to  produce  new  individuals. 
But  what  is  the  physiological  explanation  of  this  adjustment,  and  how  has 
it  arisen  in  process  of  evolution  ?  Spencer  has  elsewhere  enlarged  upon  the 
proposition,  which  we  have  already  illustrated,  that  genesis  in  all  its  forms 


286  THE    EVOLUTION    OF    SEX. 

is  a  process  of  disintegration,  and  is  thus  essentially  opposed  to  that  process 
of  integration  which  is  one  element  of  individual  evolution.  The  matter 
and  energy  supplied  for  the  young  organism  represent  so  much  loss  for 
the  parent  ;  while,  conversely,  the  larger  the  amount  of  matter  and  energy 
consumed  by  the  functional  actions  of  the  parent,  the  less  must  be  the 
amount  remaining  for  those  of  the  offspring.  The  disintegration  which 
constitutes  genesis  may  be  complete  or  partial,  and  in  the  latter  case  the 
parent,  having  reached  considerable  bulk  and  complexity  before  reproduc- 
tion sets  in,  may  survive  the  process.  In  the  same  way,  individual  evolution 
may  be  expressed  in  bulk,  in  structure,  in  amount  or  variety  of  action,  or  in 
combinations  of  these ;  yet,  in  any  case,  this  progress  of  each  individuality 
must  correspondingly  retard  the  establishment  of  the  new  ones. 

While  in  the  first  portion  of  the  argument,  then,  it  was  shown  that  a 
species  cannot  be  maintained  unless  self-preservative  and  reproductive 
power  vary  inversely,  it  is  now  evident  that,  irrespective  of  an  end  to  be 
subserved,  these  powers  cannot  do  other  than  vary  inversely,  and  the  one 
a /;7"(?;7  principle  is  thus  seen  to  be  the  obverse  of  the  other.  And  if  we 
group  under  the  term  individuation  all  those  race-preservalive  processes  by 
which  individual  life  is  completed  and  maintained,  and  extend  the  term 
genesis  to  include  all  those  processes  aiding  the  formation  and  perfecting  of 
new  individuals,  the  result  of  the  whole  argument  may  be  tersely  expressed 
in  the  formula, — Individuation  and  Genesis  vary  inversely.  And  from  this 
conception  important  corollaries  open  ;  thus,  other  things  equal,  advancing 
evolution  must  be  accompanied  by  declining  fertility  ;  again,  if  the  diffi- 
culties of  self-preservation  permanently  diminish,  there  will  be  a  permanent 
increase  in  the  rate  of  multiplication,  and  conversely. 

In  attempting  the  inductive  verification  of  these  a  priori  inferences, 
practical  difficulties  arise,  owing  to  the  high  complexity  of  each  of  our  two 
sets  of  factors  and  the  independent  variability  of  their  details,  and  thus  the 
total  cost  of  individuation  and  of  genesis  alike  is  hard  of  estimation  and 
comparison.  For  this  purpose,  however,  there  are  successively  to  be  in- 
vestigated,— (i)  the  antagonism  between  growth  and  genesis,  sexual  and 
asexual  ;  (2)  that  l)etvveen  development  and  genesis  ;  (3)  that  between  ex- 
penditure and  genesis  ;  and  (4)  the  coincidence  between  high  nutrition  and 
genesis.  It  is  impossible  to  summarise  the  wealth  of  evidence  drawn  from 
a  wide  survey  of  the  animal  and  vegetable  world  contained  in  the  chapters 
devoted  to  those  various  heads,  but  attention  may  be  called  to  the  last  and 
most  obscure  of  these.  It  is  indeed  evident  a  priori  that,  if  the  cost  of 
individuation  be  once  provided  for,  a  higher  nutrition  will  render  possible 
a  greater  propagation,  sexual  or  asexual,  and  this  may  be  abundantly  veri- 
fied by  observation  and  experiment.  Witness  the  case  of  aphides,  in  which 
the  rale  of  parlhenogenetic  reproduction  is  found  to  be  directly  proportional 
to  temperature  and  food-supply;  or,  again,  that  of  domestic  animals,  such  as 
the  sheep,  whose  fertility  is  in  direct  relation  to  richness  of  pasture  and 
warmth  of  climate  ;  or,  finally,  and  most  obviously  of  all,  that  of  field  or 
fruit  crops,  upon  which  the  influence  of  increased  liberality  of  manuring 
will  not  be  disputed.  Yet  it  is  sometimes  maintained,  for  both  plants  and 
animals,  that  overfeeding  checks  increase,  while  limited  nutriment  stimu- 
lates it  ;  and  to  support  this  view  there  are  cited  such  cases  as  that  of  the 
barrenness  of  a  very  luxuriant  plant,  and  the  fruitfulness  which  appears  on 
its  depletion.  But  if  this  objection  really  held,  manuring  would  in  all  cases 
be  inexpedient,  instead  of  only  in  plants  where  the  growth  of  sexless  axes 


LAWS    OF    MULTIPLICATION. 


287 


is  still  too  luxuriant  ;  and  a  tree  which  has  borne  a  heavy  crop  should,  by 
this  depletion,  bear  again  yet  more  heavily,  instead  of  being  more  or  less 
barren  next  year  unless  manured.  Or  the  difficulty  may  also  be  met  by 
interpreting  such  vegetative  luxuriance,  not  as  a  case  of 
higher  individuation  at  all,  but  simply  as  a  case  of  asexual 
multiplication  of  secondary  axes  ;  or  again,  and  perhaps 
most  simply,  by  regarding  the  appearance  of  sexual  re- 
production on  depletion  simply  as  a  case  of  the  previously 
demonstrated  antagonism  between  genesis  and  growth. 

But  again,  since  fatness  is  associated  with  sterility,  it 
is  often  argued  that  high  feeding  is  unfavourable  to  gene- 
sis. Obesity,  however,  is  now  known  to  be  associated 
with  imperfect  assimilation,  with  physiological  impoverish- 
ment or  degeneration, — by  no  means  with  that  constitu- 
tional wealth  which  is  favourable  to  fertility.  If,  in  short, 
we  bear  in  mind  that  truly  high  nutrition  means  only  due 
abundance  of,  and  due  proportion  among,  all  the  sub- 
stances which  the  organism  rec^uires,  and  that  their  per- 
fect assimilation  by  the  organism  is  also  needful,  such 
objections  to  the  generalisation  not  only  disappear,  but 
such  a  phenomenon  as  the  coincidence  of  returning  fer- 
tility with  disappearing  obesity  affords  a  confirmatory 
argument. 

Organisms  having  aberrant  modes  of  life  are  next  ap- 
pealed to  for  crucial  evidence  bearing  on  these  general 
doctrines.  Thus,  turning  to  vegetable  and  animal  para- 
sites, which  combine  superabundant  nutrition  with  greatly 
diminished  expenditure,  the  enormous  fertility  exhibited 
by  all  such  forms  is  seen  to  be  the  necessary  correlative 
of  such  a  state  of  nutrition  and  expenditure,  and  not 
merely  an  acquired  adaptation  to  their  peculiar  difficulties 
of  survival.  The  reversion  exhibited  by  so  many  species 
(especially  among  the  higher  arthropods,  e.g..  Aphis, 
Cecidoinyia)  from  sexual  reproduction  to  primitive  forms 
of  genesis,  is  explained  by  pointing  out  that  such  species 
are  peculiarly  situated  in  ol)taining  abundant  food  with 
little  exertion.  Among  bees,  ants,  and  termites  alike, 
the  enormous  fertility  of  the  inactive  and  highly  nourished 
queen-mother  are  obviously  also  cases  in  point. 

The  inverse  variation  of  genesis  with  individuation  has 
now  been  demonstrated  inductively  as  well  as  deductively, 
and  that  for  each  element  of  the  latter  (growth,  develop- 
ment, or  activity).  Yet  before  discussing  its  application 
to  the  problems  of  the  multiplication  of  the  human  species, 
two  points  remain, — a  question  has  to  be  answered,  and 
a  qualification  made.  The  question,  only  partially 
answered  in  course  of  the  preceding  argument,  is.  How  is 
the  ratio  between  individuation  and  genesis  established  in 
each  special  case?  and  the  answer  is,  By  natural  selec- 
tion. This  may  determine,  whether  the  quantity  of 
matter  spared  from  individuation  for  genesis  be  divided  into  many  small  ova 
or  a  few  larger  ones  ;  whether  there  shall  be  small  broods  at  short  intervals. 


A  species  of  Onion 
with  asexual  vege- 
tative bulbils  {b) 
among  the  flowers 
{a). 


288  THE    EVOLUTION    OF    SEX. 

or  larger  broods  at  longer  intervals  ;  or  whether  there  shall  be  many  unpro- 
tected offspring,  or  a  few  carefully  protected  by  the  parent.  Again,  survival 
of  the  fittest  has  a  share  in  determining  the  proportion  of  matter  subtracted 
from  individuation  for  genesis.  Yet  this  operation  of  natural  selection  goes 
on  strictly  under  the  limits  of  the  antagonism  above  traced. 

The  needed  qualification  arises  on  introducing  the  conception  of  evolu- 
tionary change.  If  time  be  left  out  of  account  as  hitherto, — or,  what  is  the 
same  thing,  if  all  the  species  be  viewed  as  permanent, — the  inverse  ratio 
between  individuation  and  genesis  holds  absolutely.  But  each  advance  in 
individual  evolution  (it  matters  not  whether  in  bulk,  in  structure,  or  in 
activities)  implies  an  economy  ;  the  advantage  must  exceed  the  cost,  else  it 
would  not  be  perpetuated.  The  animal  thus  becomes  physiologically 
richer  ;  it  has  an  augmentation  of  total  wealth  to  share  between  its  in- 
dividuation and  its  genesis.  And  thus,  though  the  increment  of  individua- 
tion tends  to  produce  a  corresponding  decrement  of  genesis,  this  latter  will 
be  somewhat  less  than  accurately  proportionate.  The  product  of  the  two 
factors  is  greater  than  before  ;  the  forces  preservative  of  race  become 
greater  than  the  forces  destructive  of  race,  and  the  species  spreads.  In 
short,  genesis  decreases  as  individuation  increases,  yet  not  quite  so  fast. 

Hence  every  type  that  is  best  adapted  to  its  conditions — every  higher 
type — has  a  rate  of  multiplication  that  ensures  a  tendency  to  predominate. 
For  though  the  more  evolved  organism  is  the  less  fertile  absolutely,  it  is 
the  more  fertile  relatively. 

The  whole  generalisation  admits  of  the  simplest  graphic 
illustration.      For    if  the    line    AB    represents    the    aggregate 

C 

A ! B 

matter  or  energies,  the  structures  or  the  functions,  of  the 
organism,  of  which  AC  denotes  the  amount  devoted  to  in- 
dividuation and  CB  to  reproduction,  the  inverse  variation  of 
AC  to  CB  is  obvious,  as  also  if  AC  and  CB  represent  the 
psychological  obverse  of  these  two  classes  of  function.  Nor 
does  an  increase  in  total  energy  modify  this,  as  when  the 
stronger  members  of  a  species  frequently  also  exhibit  greater 
reproductive  power;  for  if  in  one  case  AB  =  2o,  of  which 
C13  =  4,  and  in  another  AB  =  25,  CB  may  become  5  without 
any  rise  of  reproductive  ratio,  since  0^7  =  WV-  ^^ut  if  the  species 
be  evolving,  the  advance  in  individuation  implies  a  certain 
economy,  of  which  a  share  may  go  to  diminish  the  decrement 
to  genesis,  as  above  explained. 

i^  4.  Spencer'' s  Application  of  his  Results  to  Man. — In  ex- 
tending this  hard-won  generalisation  to  the  case  of  man,  the 
concomitance  of  all  but  highest  total  individuation  with  all  but 
lowest  rate  of  multiplication  (the  enormous  bulk  of  the  elephant 
involving  a  yet  greater  deduction  from  genesis)  is  at  once 
apparent.       Comparing    different    races    or    nations,    or    even 


LAWS    OF    MULTIPLICATION.  289 

different  social  castes  or  occupations,  the  same  holds  good ; 
while  the  prevalence  of  high  multiplication  in  races  of  which 
the  nutrition  is  in  obvious  excess  over  the  expenditure  is  also 
evident,  witness  the  Boers  or  French  Canadians,  Such  an 
apparent  difficulty  as  that  of  the  Irish,  in  whom  rapid  multipli- 
cation occurs  despite  poor  food,  is  accounted  for  by  the  re- 
latively low  expenditure  in  obtaining  it  (since  the  "  law  of 
diminishing  return  "  implies  its  converse  for  diminishing  labour), 
though,  no  doubt,  also  in  part  by  the  habit  of  early  marriage,  if 
not  by  some  measure  of  lowered  individuation  as  well.  The 
main  position  being  established,  Spencer  proceeds  to  discuss 
the  question  of  human  population  in  the  future,  and  insists 
strongly  on  the  importance  of  pressure  of  population,  which  he 
regards  as  the  main  incentive  to  progress  alike  in  past,  present, 
and  future.  Reviewing  the  possibilities  of  progress  in  bulk, 
complexity  of  structure,  multiplication  and  variation  of  func- 
tion, he  concludes  that  the  more  complete  moving  equilibrium, 
and  more  perfect  correspondence  between  organism  and 
environment,  which  such  evolution  involves,  must  take  place 
mainly  in  the  direction  of  psychical  development.  Yet  this 
development,  while  stimulated  by  pressure  of  population,  con- 
stantly tends  to  diminish  the  rate  of  fertility ;  in  other  words, 
this  cause  of  progress  tends  to  disappear  as  it  achieves  its  full 
effect.  The  acute  pressure  of  population,  with  its  attendant 
evils,  thus  tends  to  cease  as  a  more  and  more  highly  individu- 
ated race  busies  itself  with  its  increasingly  complex  yet  normal 
and  pleasurable  activities,  its  rate  of  reproduction  meanwhile 
descending  towards  that  minimum  required  to  make  good  its 
inevitable  losses. 

§  5.  Summary  of  the  Population  Question. — The  general 
question,  so  far  as  yet  developed,  may  now  be  conveniently 
summarised  in  the  accompanying  tabular  form.  Here  the 
stage  of  knowledge  reached  by  each  author,  together  with 
any  practical  applications  therefrom  deduced,  may  be  read 
horizontally,  while  the  historic  development  of  each  separate 
line  of  conceptions  may  be  traced  vertically. 

From  such  a  summary,  brief  as  it  is,  the  main  steps  in  the 
development  of  our  knowledge  are  clear  enough,  but  a  deeper 
analysis  is  required  before  final  exposition  or  complete  appli- 
cation is  possible.  Nor,  when  we  note  how  vast  the  progress 
of  science   through    the   advance  in    precision  and  extension 


290 


THE    EVOLUTION    OF    SEX. 


effected  upon  the  conception  of  Maltbus"^  by  Darwin,  will  the 
utility  of  such  increasing  elaboration  be  disputed.  Thus  the 
full  inductive  verification  of  Spencer's  law  involves  a  detailed 


Author. 

Development  of  Theory  of  Population. 

Practical 

Action 
Deduced. 

I. 

Non -bio- 
logical 
writers 
(prede- 
cessors 
and  op- 
ponents 
of  Mal- 
thus). 

Increase  of  population 
does  not  tend  to  out- 
run subsistence. 

■ 

II. 

Malthus. 

1798. 

Increase  of  population 
tends  to  outrun   that 
of  subsistence. 

But  meets  checks : 

A.  Positive. 

B.  Preventive. 

To    avoid     A, 
adopt  B. 

III. 

Darwin. 

1859. 

Do. 

Hence      struggle 
for  existence  : 

A.  Natural 
selection. 

B.  Artificial 
selection. 

Leading  to 
evolution. 

Laissez-faire, 
i.e.,    on    ac- 
count of  ad- 
vantage     to 
species  from 
A,  avoid  B. 

IV. 

Spencer. 
1852-66. 

Do. 
Rate   of  multiplication 
investigated    for    dif- 
ferent    species,     and 
shown  to  vary  inverse- 
ly as  individuation. 

Do. 

Do. 
Also     lead- 
ing       to 
e  v  0  1  u- 
tion        of 
species. 

Do. 
[Individiiaie.} 

comparison  of  the  rates  of  reproduction  of  each  group  of 
organic  species,  with  their  observed  degree  of  individuation 
(first  in  each  of  its  factors,  and  finally  in  their  sum),  devia- 
tions from  the  inverted  symmetry  of  the  theoretic  curves 
(see  fig.  opposite)  having  to  be  separately  discussed.  Natural 
selection  also  requires  a  yet  deeper  analysis ;  the  limits  and 
possibilities  of  artificial  selection  are  but  little  known,  while 


*  It  is  also  interesting  to  compare  Malthus's  view  of  population,  tend- 
ing to  increase  in  geometrical  proportion  and  substance  only  in  arithmetical, 
with  Spencer's  demonstration  of  the  limit  of  growth  already  summarised 
(see  p.  220),  the  more  so  when  we  bear  in  mind  that  reproduction  is  dis- 
continuous growth.  The  precise  statement  of  Malthus  becomes  confirmed, 
as  regards  the  cell,  if  not  the  cell  aggregate. 


LAWS    OF    MULTIPLICATION. 


291 


a  theory  of  variation  is  still  far  from  agreed  upon.  If  how- 
ever we  bear  in  mind  that  the  amount  of  evolution  in  given 
time  is  but  small,  our  knowledge  seems  not  insufficient  for  the 
practical  deductions  which  are  so  pressingly  demanded ;  yet  it 
is  here  that  the  most  serious  disagreement  has  prevailed. 
Thus  the  Malthusian  position  is  obviously  inadecjuate,  in  not 
allowing  for  the  Darwinian  one ;  yet  the  converse  also  is 
undeniable,  for  the  position  of  laissez-faire^  upon  which  Darwin 
and  Spencer  alike  take  their  stand,  not 
only  almost  ignores  the  wellbeingof  the 
individual  in  considering  the  advance- 
ment of  the  species,  but  is  even  then 
too  optimistic,  since  it  not  only  fails 
to  accelerate  the  progressive  evolution 
which  is  alone  considered,  but  also  fails 
to  provide  against  the  equal  possibility 
of  degenerative  change.  Are  we  then 
simply  to  return  to  the  somewhat  crude 
proposals  and  excessive  hopes  for  the 
increase  of  individual  wellbeing  due  to 
Malthus  or  his  followers,  based  too  as 
these  have  been  on  imperfect  pre- 
Spencerian  knowledge  ? 

The  answer  is  not  far  to  seek, — it 

lies    in    the    generalisation    above    eStab-    LeUhe  perpendiculars  above  the 

lished  ;  yet  it  is  remarkable  that  Mr 
Spencer,  after  not  only  establishing  the 
inverse  variation  of  individuation  and 
genesis  among  species  in  general,  but 
even  showing  for  the  human  species  in 
particular  that  it  is  essentially  upon 
increase  of  the  psychical  activities  that 
the  increased  mdividuation  and  dimin- 
ished genesis  of  the  future  must  depend, 
should  not  have  proceeded  to  a  fuller  application.  For  unless 
the  main  generalisation  be  abandoned,  it  is  obvious  that  the 
progress  of  the  species  and  of  the  individual  alike  is  secured  and 
accelerated  whenever  action  is  transferred  from  the  negative 
side  of  merely  seeking  directly  to  repress  genesis,  to  the 
positive  yet  indirect  side  of  proportionally  increasing  individua- 
tion. This  holds  true  of  all  species,  yet  most  fully  of  man, 
since   that   modification    of  psychical   activities    in   which   his 


line  A  B  denote  the  increasing 
degree  of  total  individuation 
of  a  series  of  forms  i,  2,  3,  4, 
5,  6  (say  Worm,  Fish,  Frog, 
Bird,  Man,  Elephant),  and 
similarly  let  the  perpendicu- 
lars to  C  I)  represent  the  rate 
of  multiplication  of  the  same 
forms  ;  the  curves  joining 
these  two  series  of  points 
respectively  illustrate  by  their 
inverted  symmetry  the  inverse 
ratio  of  individuation  and 
genesis. 


292  THE    EVOLUTION    OF    SEX. 

evolution  essentially  lies,  is  par  excellence  and  increasingly  the 
respect  in  which  artificial  comes  in  to  replace  natural  selection. 
Without  therefore  ignoring  the  latter,  or  hoping  ever  wholly 
to  escape  from  the  iron  grasp  of  nature,  we  yet  have  within  our 
power  more  and  more  to  mitigate  the  pressure  of  population, 
and  that  without  any  sacrifice  of  progress,  but  actually  by 
hastening  it.  vSince  then  the  remedy  of  pressure  and  the  hope 
of  progress  alike  lie  in  advancing  individuation,  the  course  for 
practical  action  is  clear, — it  is  in  the  organisation  of  these 
alternate  reactions  between  bettered  environment  (material, 
mental,  social,  moral)  and  better  organism  in  which  the  whole 
evolution  of  life  is  defined,  in  the  conscious  and  rational 
adjustment  of  the  struggle  into  the  culture  of  existence. 

The  practical  corollaries  of  the  Malthusian  view  are  celibacy, 
late  marriage,  and  moral  control ;  the  objections  are  vice,  in- 
creased mortality  in  childbirth,  and  the  present  low  evolution 
of  our  moral  nature.  The  practical  corollary  of  the  Darwinian 
doctrine  is  virtually  nil ;  the  objection,  that  the  survival  of 
what  we  consider  the  best  types  is  doubtful,  and  that  the 
survival  of  the  fit  is  apt  to  be  cruel.  The  practical  corollaries  of 
the  Spencerian  principle,  although  Mr  Spencer  can  hardly  be 
said  to  have  insisted  upon  these,  are  individuate  and  educate. 
The  objection  is,  that  the  pressure  of  population  is  already  felt, 
and  that  individuation  is  a  matter  of  centuries.  Furthermore, 
the  effect  of  education,  for  instance  in  reducing  sexuality,  will 
tell  most  where  it  is  least  wanted,  viz.,  among  the  best  types. 

We  are  therefore  bound  to  include,  as  a  continuation  of  the 
above  table,  the  amendment  of  some  of  the  most  thoughtful  ex- 
ponents of  what  is  generally  called  neo-Malthusian  doctrine. 
This  advocates  the  use  of  artificial  preventive  checks  to  fer- 
tilisation. Discussion  of  this  proposal  is  at  present  difiicult, 
because  of  the  comparative  absence  of  distinctly  expressed 
opinion  on  the  part  of  medical  experts,  and  because  of  strong 
superficial  prejudices,  not  only  against  the  scheme,  but  against 
its  discussion.  These  prejudices  are,  however,  dying  out,  and 
that  is  well,  for  they  do  nothing  but  obscure  appreciation  alike 
of  the  merits  and  demerits  of  the  doctrine.  An  increasing 
realisation  of  the  plain  facts  of  reproduction  and  population 
must  rapidly  exterminate  the  persistently  theological  absurdities 
which  people  utter,  if  they  do  not  believe  on  the  subject.  The 
vague  feeling  that  control  of  fertilisation  is  "  interfering  with 
nature,"    in    some    utterly   unwarrantable    fashion,    cannot    be 


LAWS    OF    MULTIPLICATION.  293 

consistently  stated  by  those  who  live  in  the  midst  of  our  highly 
artificial  civilisation.  The  strongest  prejudice  seems  to  be 
based  in  a  moral  cowardice,  which  gauges  a  scheme  by  its 
"  respectability,'^  while  even  more  culpable  is  that  consciously 
or  unconsciously  derived  from  the  profitableness  to  the 
capitalist  classes  of  unlimited  competition  of  cheap  unskilled 
labour.  For  never  did  the  proletariat  more  literally  deserve  its 
name  than  since  the  advent  of  the  factory  period,  their  rapid 
and  degenerative  increase,  indeed,  primarily  representing  "  the 
progress  of  investments." 

The  general  attitude  of  the  modern  Malthusian  may  first  of 
all  be  roughly  indicated  by  quoting  the  mottoes  which  head 
the  organ  of  their  league.  "  To  a  rational  being,  the  prudential 
check  to  population  ought  to  be  considered  as  equally  natural 
with  the  check  from  poverty  and  premature  mortality"  (Malthus, 
1806).  "  Little  improvement  can  be  ex])ected  in  morality  until 
the  production  of  large  families  is  regarded  in  the  same  light  as 
drunkenness,  or  any  other  physical  excess  "  (John  Stuart  Mill, 
1872).  "Surely  it  is  better  to  have  thirty-five  millions  of 
human  beings  leading  useful  and  intelligent  lives,  rather  than 
forty  millions  struggling  painfully  for  a  bare  subsistence " 
(Lord  Derby,  1879).  Starting  from  the  familiar  induction 
that  "  population  has  a  constant  tendency  to  outrun  the 
meajis  of  subsistence,"  they  recognise  in  this  over-population 
"  the  most  fruitful  source  of  pauperism,  ignorance,  crime, 
and  disease."  To  counteract  this  there  are  checks,  posi- 
tive or  life-destroying  on  the  one  hand,  prudential  or  birth- 
preventing  on  the  other.  "  The  positive  or  life-destroying 
checks  comprehend  the  premature  death  of  children  and  adults 
by  disease,  starvation,  war,  and  infanticide."  As  these  positive 
checks  are  happily  reduced  with  the  progress  of  society, 
attention  must  be  concentrated  on  the  other  side.  "  This 
consists  in  the  limitation  of  offspring  by  abstention  from 
marriage,  or  by  prudence  after  marriage."  But  as  to  the  first, 
prolonged  abstention  from  marriage,  as  advocated  by  Malthus, 
this  is  '^  productive  of  many  diseases,  and  of  much  sexual  vice," 
while  "  early  marriage,  on  the  contrary,  tends  to  secure  sexual 
purity,  domestic  comfort,  social  happiness,  and  individual 
health."  The  check  that  remains  to  be  advocated  is  thus 
"  prudence  after  marriage,"  and  by  this  the  neo-Malthusians 
most  distinctly  mean  attention  to  methods  which  will  secure 
that  sexual  intercourse  be  not  followed  by  fertilisation.     For 


294  I'HE    EVOLUTION    OF    SEX. 

the  details  of  the  various  methods,  we  must  refer  to  the 
Malthusian  hterature ;  but  a  brief  outhne  is  imperative,  even 
for  an  approximate  understanding  of  the  problem. 

{a.)  Thus  we  have  the  suggestion  that  intercourse  should  be 
limited  to  the  relatively  infertile  period  most  remote  from 
menstruation,  when  conception  may  indeed  occur,  but  with 
less  probability  than  at  other  periods.  Although  gynaecologists 
are  disagreed  as  to  the  degree  of  this  probability,  there  can  be 
little  doubt  that  such  limitation  would  have  a  useful  influence, 
although  in  itself  confessedly  incomplete.  The  so-called 
artificiality  of  control  is  here  reduced  to  a  minimum,  and  the 
suggestion  is  obviously  in  harmony  with  that  increased 
temperance  which  all  must  allow  to  be  desirable. 

(b.)  In  the  second  place,  there  are  methods  employed  by 
the  males,  such  as  that  of  withdrawal  before  the  emission 
of  the  seminal  fluid,  a  habit  common  enough  both  in  savage 
and  civilised  communities.  Fertilisation  is  in  this  way  ab- 
solutely prevented,  but  apart  from  a  more  general  objection  to 
be  afterwards  emphasised,  such  a  practice  is  maintained  by 
some  to  be  injurious  to  the  male,  and  yet  more  to  the  female. 
Moreover,  although  the  risks  of  over-population  and  female 
exhaustion  by  child-bearing  are  here  minimised,  there  is  still 
risk  of  male  exhaustion. 

(r.)  Thirdly,  although  again  under  the  severe  criticism  of 
some  of  the  medical  experts,  there  are  means  employed  by  the 
females,  for  securing  by  means  of  pessaries  that  the  spermatozoa 
do  not  come  into  contact  with  the  ovum,  or  by  means  of  washes 
that  the  male  elements  are  rendered  ineffectual.  In  reply  to 
the  medical  objections  to  both  these  methods  of  artificial  check, 
it  is  answered  {a)  that  it  may  in  many  cases  be  necessary  to 
choose  between  two  evils,  of  which  the  risk  involved  in  the 
artificial  check  may  be  much  less  than  that  involved  in  con- 
tinued child-bearing  ;  (/')  that  it  is  hardly  a  fair  argument  as 
yet  to  urge  that  the  proposed  checks  of  neo-Malthusianism 
are  fraught  with  danger.  As  to  the  popularly  supposed  pre- 
ventive check  of  })rolonged  nursing  one  baby  in  the  hope  of 
thereby  preventing  a  new  conception,  it  is  necessary  to  em- 
phasise that  nursing  does  not  effect  this,  and  that  the  prolonga- 
tion of  the  lacteal  function  and  diet  beyond  their  natural  limits 
is  seriously  injurious  alike  to  mother  and  off'spring. 

Even  recognising  some  of  these  objections,  the  neo-Malthu- 
sians  urge  the  number  of  distinct  advantages, — the  reduction 


LAWS    OF    MULTIPLICATION.  295 

of  the  present  rapid  rate  of  increase ;  the  possibihty  of  earher 
marriages,  and  a  probable  diminution  of  vice ;  an  increase  in 
the  fitness  of  the  race  by  lessening  the  propagation  of  unfit 
types  and  the  exhaustion  of  the  mothers  by  too  frequent  child- 
bearing.  Supposing,  again,  the  general  adoption  of  the  pro- 
posal, the  neo-Malthusians  insist  upon  the  possibility  of  a 
heightened  standard  of  comfort  among  the  poorer  members 
of  the  community,  and  the  removal  of  obstacles  to  marriage 
which  stand  in  the  way  of  those  who  ought  to  marry  but  ought 
not  to  be  parents. 

Without  urging  medical  objections  above  referred  to, — 
for  in  regard  to  the  discussion  of  these,  professional  experts 
must  bear  the  responsibility, — we  must  emphasise  several 
counter-arguments.  Thus  it  has  been  maintained,  though  with 
no  great  degree  of  certitude,  that  a  proposal  involving  some 
deliberate  and  controlled  action  would  tend  to  be  adopted 
most  where  least  wanted,  viz.,  among  the  more  individuated 
types,  whose  numbers  would  in  consequence  be  proportionately 
reduced.  The  diminished  rate  of  increase,  which  is  the  most 
obvious  social  result  of  the  extensive  adoption  of  neo- 
Malthusian  practices,  has  long  been  known  to  the  student  of 
population ;  and  in  some  countries,  particularly  France, — 
although  here,  no  doubt,  to  some  extent  the  result  of  peculiarly 
high  individuation, — is  a  recognised  national  danger,  especially 
since  the  diminished  population,  in  being  largely  freed  from  the 
normal  acuteness  of  the  struggle  for  existence,  loses  many  of 
the  advantages  of  this  as  well. 

The  statistician  will  doubtless  long  continue  his  fashion  of 
confidently  estimating  the  importance  and  predicting  the  sur- 
vival of  populations  from  their  quantity  and  rate  of  reproduction 
alone  ;  but  at  all  this,  as  naturalists  we  can  only  scoff.  Even 
the  most  conventional  exponent  of  the  struggle  for  existence 
among  us  knows,  with  the  barbarian  conquerors  of  old,  that  "the 
thicker  the  grass,  the  easier  it  is  mown;"  that  "the  w^olf  cares 
not  how  many  the  sheep  may  be."  It  is  the  niost  individuated 
type  that  prevails  in  spite,  nay,  in  another  sense,  positively 
because  of  its  slower  increase  ;  in  a  word,  the  survival  of  a 
species  or  family  depends  not  primarily  upon  quantity,  but 
upon  quality.  The  future  is  not  to  the  most  numerous  popu- 
lations, but  to  the  most  individuated.  And  as  w^e  increas- 
ingly see  that  natural  history  must  be  treated  primarily  from 
the  standpoint  of  the  species-regarding  sacrifice   rather   than 


296  THE    EVOLUTION    OF    SEX. 

from  that  of  the  individual  struggle,  we  see  the  importance  of 
the  general  neo-Malthusian  position,  despite  the  risks  which  the 
particular  modes  of  its  practice  may  involve. 

Apart  from  the  pressure  of  population,  it  is  time  to  be  learn- 
ing (i)  that  the  annual  childbearing  still  so  common,  is  cruelly 
exhaustive  to  the  maternal  life,  and  this  often  in  actual  duration 
as  well  as  quality ;  (2)  that  it  is  similarly  injurious  to  the 
standard  of  offspring ;  and  hence  (3)  that  an  interval  of  two 
clear  years  between  births  (some  gynaecologists  even  go  as  far 
as  three)  is  due  alike  to  mother  and  offspring.  It  is  time  there- 
fore, as  we  heard  a  brave  parson  tell  his  flock  lately,  "  to  have 
done  with  that  blasphemous  whining  which  constantly  tries  to 
look  at  a  motherless  "  (ay,  or  sometimes  even  fatherless)  "  crowd 
of  puny  infants  as  a  dispensation  of  mysterious  providence." 
Let  us  frankly  face  the  biological  facts,  and  admit  that  such 
cases  usually  illustrate  only  the  extreme  organic  nemesis  of 
intemperance  and  improvidence,  and  these  of  a  kind  far  more 
reprehensible  than  those  actions  to  which  common  custom 
applies  the  names,  since  they  are  species-regarding  vices,  and 
not  merely  self-regarding  ones,  as  the  others  at  least  primarily 
are.  To  realise  the  social  consequences  of  sexual  intemperance 
is  enough  to  obviate  any  hasty  criticism  of  neo-Malthusianism, 
whatever  conclusion  may  be  arrived  at  as  to  its  sufficiency. 

It  is  time,  however,  to  point  out  the  chief  weakness  in  neo- 
Malthusian  proposals,  which  are  at  one  in  allowing  the  gratifica- 
tion of  sexual  appetites  to  continue,  aiming  only  at  the  preven- 
tion of  the  naturally  ensuing  parentage.  To  many  doubtless 
the  adoption  of  a  method  which  admits  of  the  egoistic  sexual 
pleasures,  without  the  responsibilities  of  childbirth,  would  mul- 
tiply temptations.  Sexuality  would  tend  to  increase  if  its  respon- 
sibilities were  annulled  ;  the  proportion  of  unchastity  before 
marriage,  in  both  sexes,  could  hardly  but  be  augmented ; 
while  married  life  would  be  in  exaggerated  danger  of  sinking 
into  "  monogamic  prostitution."  On  the  other  hand,  it  seems 
probable  that  the  very  transition  from  unconscious  animalism 
to  deliberate  prevention  of  fertilisation,  would  tend  in  some  to 
decrease  rather  than  increase  sexual  appetite. 

It  seems  to  us,  however,  essential  to  recognise  that  the  ideal 
to  be  sought  after  is  not  merely  a  controlled  rate  of  increase, 
but  regulated  married  lives.  Neo-Malthusianism  might  secure 
the  former  by  its  more  or  less  mechanical  methods,  and  there 
is  no  doubt  that  a  limitation  of  the  family  would  often  increase 


LAWS    OF    MULTIPLICATION.  297 

the  happiness  of  the  home ;  but  there  is  danger  lest,  in  re- 
moving its  result,  sexual  intemperance  become  increasingly 
organic.  We  would  urge,  in  fact,  the  necessity  of  an  ethical 
rather  than  of  a  mechanical  "  prudence  after  marriage,"  of  a 
temperance  recognised  to  be  as  binding  on  husband  and  wife 
as  chastity  on  the  unmarried.  When  we  consider  the  inevit- 
able consequences  of  intemperance,  even  if  the  dangers  of  too 
large  families  be  avoided,  and  the  possibility  of  exaggerated 
sexuality  becoming  cumulative  by  inheritance,  we  cannot  help 
recognising  that  the  intemperate  pair  are  falling  towards  the 
ethical  level  of  the  harlots  and  profligates  of  our  streets. 

Just  as  we  would  protest  against  the  dictum  of  false  physi- 
cians who  preach  indulgence  rather  than  restraint,  so  we  must 
protest  against  regarding  artificial  means  of  preventing  fertilisa- 
tion as  adequate  solutions  of  sexual  responsibility.  After  all,  the 
solution  is  primarily  one  of  temperance.  It  is  no  new  nor 
unattainable  ideal  to  retain,  throughout  married  life,  a  large 
measure  of  that  self-control  which  must  always  form  the  organic 
basis  of  the  enthusiasm  and  idealism  of  lovers.  But  as  old 
attempts  at  the  regulation  of  sexual  life  have  constantly  fallen 
from  a  glowing  idealism  into  pallor  or  morbidness,  it  need 
hardly  be  said  that  the  same  fate  w'ill  ever  more  or  less 
befall  the  endeavour  after  temperance,  so  long  as  that  lacks 
the  collaboration  of  other  necessary  reforms.  We  need  a 
new  ethic  of  the  sexes ;  and  this  not  merely,  or  even  mainly, 
as  an  intellectual  construction,  but  as  a  discipline  of  life ; 
and  we  need  more.  We  need  an  increasing  education  and 
civism  of  women, — in  fact,  an  economic  of  the  sexes  very 
different  from  that  nowadays  so  common,  which,  while  attack- 
ing the  old  co-operation  of  men  and  women  because  of  its 
manifest  imperfections,  only  offers  us  an  unlimited  and  far 
more  mutually  destructive  industrial  competition  between  them 
instead.  The  practical  problems  of  reproduction  become  in 
fact,  to  a  large  extent,  those  of  improved  function  and  evolved 
environment ;  and  limitation  of  population,  just  as  we  are  be- 
ginning to  see  the  cure  of  the  more  individual  forms  of  intem- 
perance, is  primarily  to  be  reached,  not  solely  by  individual 
restraint,  but  by  a  not  merely  isolated  and  individual,  but  aggre- 
gate and  social,  reorganisation  of  life,  work,  and  surroundings. 
And  while  our  biological  studies  of  course  for  the  most  part 
only  point  the  way  towards  deeper  social  ones,  they  afford  also 
one  luminous  principle  towards  their  prosecution, — that  thorough 


298  THE    EVOLUTION    OF    SEX. 

parallelism  and  coincidence  of  psychical  and  material  considera- 
tions, upon  which  moralist  and  economist  have  been  too  much 
wont  respectively  to  specialise. 

§  6.  Rate  of  Reproduction  ''^  Nil" — Sterility. — When  we  view 
reproduction  in  terms  of  discontinuous  growth, — that  is,  as  a 
phenomenon  of  disintegration, — it  is  obvious  that  complete 
integration  of  the  matter  acquired  by  the  organism  into  its  own 
bulk,  and  for  its  own  development,  precludes  reproduction, — 
that  is,  involves  sterility, — and  similarly  as  regards  the  energies 
of  the  organism.  This  is  only  a  re-statement  of  Spencer's 
generalisation  above  discussed ;  for  it  is  evident  that,  if  genesis 
vary  inversely  as  individuation,  it  must  be  suppressed  altogether 
if  individuation  becomes  complete.  The  actual  phenomena, 
however,  by  no  means  usually  admit  of  explanation  as  such 
realisations  of  the  ideal  of  evolution,  and  hence  the  cause  and 
treatment  of  sterility  mainly  pass  into  the  provinces  of  the 
experimental  naturalist  and  the  physiological  physician.  From 
the  earliest  times,  indeed,  physician  and  naturalist,  priest  and 
legislator,  alike  devoted  attention  to  the  subject ;  and  it  was 
probably  in  this  way,  as  a  recent  monographer  remarks,  that 
research  became  directed  to  the  larger  problem  of  repro- 
duction in  general.  The  general  biological  questions — 
e.g.^  the  relations  between  sterility  within  the  limits  of 
a  species  to  changes  in  the  environment,  or  that  of  sterility 
among  hybrids  —  are  extensively  discussed  in  the  copious 
literature  which  centres  around  Darwin's  Variatio?i  of  A?ii7tials 
a?id  Plants  under  Domestication ;  while  with  regard  to  the 
human  species,  an  extensive  medical  literature  of  course  exists, 
to  which  any  encyclopedia  of  medicine,  or  conveniently  the 
recent  careful  monograph  of  P.  Miiller  {Die  Ufifruchtbarkeit 
der  Ehe^  Stuttgart,  1885),  will  furnish  bibliographical  details. 


LAWS    OF    MULTIPLICATION.  299 


SUMMARY. 

1.  The  rate  of  reproduction  is  chiefly  determined  by  the  constitution  of 
the  organism  ;  the  rate  of  increase,  by  its  relations  to  the  animate  and 
inanimate  environment. 

2.  The  naturalist  has  to  thank  the  sociologist  for  directing  emphatic 
attention  to  the  laws  of  multiplication. 

3.  Summary  of  Spencer's  analysis.  Individuation  and  genesis  vary 
inversely. 

4.  In  regard  to  man,  Spencer  urges  the  importance  of  pressure  of  popu- 
lation as  an  incentive  to  progress,  and  concludes  that  man's  future  evolution 
must  continue  mainly  in  the  direction  of  psychical  development,  and  pre- 
dicts with  the  increase  of  individuation  a  diminution  of  fertility. 

5.  Predecessors  and  opponents  of  Malthus  denied  that  increase  of 
population  tended  to  outrun  subsistence  ;  Malthus  successfully  demon- 
strated his  thesis,  and  noted  the  checks  which  curbed  the  increase  ;  Darwin 
emphasised  the  advantage  of  the  pressure  and  checks  ;  Spencer  shows  the 
inverse  ratio  of  degree  of  development  and  rate  of  reproduction  ;  neo- 
Malthusians  advocate  the  use  of  artificial  preventive  checks  to  fertilisation. 
Discussion  of  these  various  generalisations  and  proposals. 

6.  Completed  imlividuation,  were  that  possible,  would  be  theoretically 
associated  with  sterility. 


LITERATURE. 

Malthus. — Theory  of  Population.      1806. 

Spencf:r. — Principles  of  Biology.     Lond.  1866. 

Geddes. — "  Reproduction,"   Ency.    Brit.  ;    and    Lecture    on    Claims     of 

Labour.     Edin.  1886. 
Drysdale. — The  Population  Question.     Lond.  1878. 
Besant. — The  Law  of  Population.     Lond.  n.d. 
Clapperton.— Scientific  Meliorism.     Lond.  1885. 


CHAPTER    XXI 

THE    REPRODUCTIVE    FACTOR    IN    EVOLUTION. 

§  I.  Ge7ieral  History  of  Evohitio7i. — The  history  of  the  doctrine 
of  evolution  is  essentially  modern  ;  for  though  the  idea  glim- 
mered before  the  minds  of  many  ancient  philosophers  from 
Empedocles  to  I>ucretius,  it  was  not  till  the  eighteenth  century 
that  naturalists  began  seriously  to  apply  the  conception  to  the 
problem  of  the  origin  of  our  fauna  and  flora.  In  thinking  of 
the  history,  it  is  necessary  to  distinguish,  on  the  one  hand,  the 
gradual  demonstration  of  the  fact  that  evolution  is  a  modal 
explanation  of  the  origin  of  organisms,  and,  on  the  other,  the 
deeper  problem  of  the  real  mechanism  of  the  process.  The 
former,  the  empirical  fact  of  evolution,  may  be  said  to  have 
been  virtually  demonstrated,  soon  after  the  middle  of  this 
century,  by  the  labours  of  Sj^encer,  Darwin,  Wallace,  Haeckel, 
and  others;  the  latter— the  real  aetiology  of  organisms,  the 
"how"  of  the  process — is  still  the  subject  of  searching  inquiry 
and  keen  debate. 

The  idea  of  evolution,  for  so  many  centuries  a  latent  germ, 
first  took  definite  shape,  so  far  as  biology  is  concerned,  in  the 
mind  of  Buffon  (1749),  who  not  only  urged  the  general  con- 
ception with  diplomatic  skill  and  powerful  irony,  but  sought  to 
elucidate  the  working  out  of  the  process.  He  illustrated  the 
influence  of  new  conditions  in  evoking  new  functions ;  showed 
how  these  in  turn  reacted  upon  the  structure  of  the  organism  ; 
and  how,  most  directly  of  all,  altered  climate,  food,  and  other 
elements  of  the  environment,  were  external  factors  in  internal 
change,  whether  for  progress  or  for  degeneration. 

Contrasted  with  Buffon  in  many  ways,  both  in  his  mode  of 
treatment  and  in  his  view  of  the  factors,  was  Erasmus  Darwin 
(1794),  the  grandfather  of  the  author  of  the  "Origin  of  Species." 
In  rhyme  and  reason,  with  all  the  humour  and  common-sense 
of  a  true  Englishman,  and  with  a  really  living  conception  of 
nature,   he    urged    the   general   conception   of  evolution,   and 


THE  REPRODUCTIVE  FACTOR  IN  EVOLUTION.      30I 

emphasised  the  organism's  inherent  power  of  self-improvement, 
the  moulding  influence  of  new  needs,  desires,  and  exertions, 
and  the  i7idirect  action  of  the  environment  in  evoking  these. 

To  Treviranus  (writing  in  1802-31) — a  biologist  too  much 
neglected  both  in  his  lifetime  and  since — organisms  appeared 
almost  indefinitely  plastic,  especially  however  under  the  direct 
influence  of  external  forces.  His  keen  analysis  of  possible 
factors  did  not  fail  to  recognise, — ^what  Brooks,  Galton,  Weis- 
mann,  and  others  have  since  elaborated, — that  the  union  of 
diverse  sexual  elements  in  fertilisation  was  in  itself  a  fountain 
of  change.  "  Every  form  of  life,"  he  says,  "  may  have  been 
produced  by  physical  forces  in  either  of  two  ways,  either  from 
formless  matter,  or  by  the  continuous  modification  of  form. 
In  the  latter  case,  the  cause  of  change  may  be  either  in  the 
influence  of  the  heterogeneous  male  j-epj'oduclive  matter  on  the 
female  germ^  or  in  the  influence  of  other  potencies  after 
generation." 

His  contemporary  Lamarck  (writing  in  1 801-9)— of  gi"eater 
posthumous  fame — fought  in  poverty  like  a  hero  for  the  evolu- 
tionary conceptions  of  his  later  years.  He  is  well  known  to 
have  emphasised  the  importance  of  changed  conditions  in 
evoking  new  needs,  desires,  and  activities,  urging  at  the  same 
time  the  perfection  wrought  upon  organs  by  increased  practice, 
and  conversely  the  degeneration  which  follows  as  the  nemesis 
of  disuse.  Evolution  seemed  to  him  to  be  due  to  the  inter- 
action of  two  fates, — an  internal  progressive  power  of  life  ;  and 
the  external  force  of  circumstances,  encountered  in  the  twofold 
struggle  with  the  inanimate  environment  and  with  living 
competitors. 

Among  the  philosoi)hers  too,  and  especially  in  the  minds 
of  those  who  had  been  disciplined  in  physical  or  historical 
investigations,  the  speculations  of  the  ancients  were  ever  taking 
fresh  form,  gaining  moreover  in  concreteness.  Thus  Kant 
viewed  the  evolution  of  species  mainly  in  terms  of  the 
mechanical  laws  of  the  organism  itself,  but  allowed  also  for 
the  influence  of  environment,  noted  the  importance  of  selection 
in  artificial  breeding,  and,  like  such  ancients  as  Empedocles 
and  Aristotle,  had  glimpses  of  the  notion  of  the  struggle  for 
existence.  The  same  idea  is  more  distinct  in  Herder's 
"Philosophy  of  History,"  where,  probably  under  Goethe's 
influence,  he  speaks  of  the  "struggle,  each  one  for  itself,  as  if 
it  were  the  only  one,"  of  the  limits  of  space,  and  of  the  gain  to 


302  THE    EVOLUTION    OF    SEX. 

the  whole  from  the  competition  of  individuals.  Oken  (1809) 
saw  the  light  of  the  evolution  idea  dancing  like  a  will-o'-the-wisp 
in  the  mist  of  his  "  Urschleim "  speculations,  and  seemed 
chiefly  to  interpret  the  organic  progress  in  terms  of  action  and 
reaction  between  the  organism  and  its  surroundings ;  while  in 
the  noble  epic  of  evolution  which  we  owe  to  his  contemporary 
Goethe,  the  adaptive  influence  of  the  environment  is  clearly 
recognised. 

Wells  in  181 3,  and  Patrick  Matthew  in  1831,  forestalled 
Darwin  in  suggesting  the  importance  of  natural  selection ;  but 
their  virtually  buried  doctrines,  however  interesting  historically, 
were  of  less  practical  importance  than  those  of  Robert 
Chambers,  the  long  unknown  author  of  the  "  Vestiges  of 
Creation  "  (1844-53).  ^^^  hypothesis  of  evolution  emphasised 
the  grow^ing  or  evolving  powers  of  the  organisms  themselves, 
which  developed  in  rhythmic  impulses  through  ascending 
grades  of  organisation,  modified  at  the  same  time  by  external 
circumstances,  which  acted  with  most  effect  on  the  generative 
system.  It  is  difficult  indeed  to  refrain  from  amusement  or 
irritation  at  the  naive  simplicity  with  which  he  evolves  a 
mammal  from  a  bird,  by  the  short  and  easy  method  of  prolong- 
ing the  period  of  uterine  life  in  favourable  nutritive  conditions; 
but  though  a  goose  could  not  so  simply  give  rise  to  a  rat,  the 
emphasis  laid  on  the  influence  of  prolonged  gestation  is  full  of 
suggestiveness,  especially  in  relation  to  the  evolution  of 
mammals.  Apart  from  his  common-sense  view  of  evolution 
as  a  process  of  continued  growing.  Chambers  deserves  to  be 
remembered  as  one  of  the  first  to  appreciate  "  the  force  of 
certain  external  conditions  operating  upon  the  parturient 
system." 

In  France,  Geoflroy  and  Isidore  St  Hilaire — father  and 
son — denied  indefinite  variations,  regarded  function  as  of 
secondary  importance,  and  laid  special  stress  upon  the  direct 
influence  of  the  environment.  To  them  it  seemed  not 
so  much  the  effort  to  fly,  as  the  (supposed)  diminished  pro- 
portion of  carbonic  acid  in  the  atmosphere,  which  had  deter- 
mined the  evolution  of  birds  from  ancient  reptiles.  A  complete 
history  of  evolution  theories,  up  to  the  publication  of  the 
"Origin  of  Species"  (1859),  would  have  to  take  account 
further  of  the  opinions  of  the  geographer  Von  Buch  and  the 
embryologist  Von  Baer,  of  Schleiden  and  Naudin,  Owen  and 
Cams,  and  many  others  ;  but  no  such  survey  is  here  our  purpose. 


THE  REPRODUCTIVE  FACTOR  IN  EVOLUTION.      303 

For  it  must  be  already  evident  from  the  above  brief  sketch 
of  representative  opinions,  that  successive  naturalists  have 
emphasised  now  one  factor  and  now  another  in  the  evolu- 
tionary process.  To  one  it  seemed  as  if  the  organism  had  a 
motor  power  of  development— often  a  metaphysical  one,  it 
must  be  allowed — within  itself,  and  that  evolution  was  to  be 
explained,  in  Topsian  fashion,  "  according  to  the  laws  of 
organic  growth  ;  "  to  another,  function  appeared  all-important, 
perfecting  organs  on  the  one  hand,  allowing  them  to  wane  in 
disuse  on  the  other ;  to  a  third,  organisms  were  seen  under 
the  hammers  of  external  forces  and  circumstances,  being  con- 
tinuously welded  in  more  and  more  perfectly  adapted  forms. 
The  organism,  its  function,  and  its  environment,  on  each  of 
the  three  factors  in  the  problem  emphasis  was  in  turn  laid. 

At  this  juncture  Darwin  elaborated  his  theory  of  "  The 
Origin  of  Species  by  means  of  Natural  Selection  and  the 
Preservation  of  Favoured  Races  in  the  Struggle  for  Life,"  and 
was  independently  and  simultaneously  corroborated  by  Alfred 
Russel  Wallace.  They  did  not  indeed  deny  a  spontaneous 
power  of  change  in  the  organism  itself,  nor  the  influence  of 
function  and  environment;  but,  without  definitely  discussing 
the  origin  of  variations,  sought  to  show  how  the  destructive  or 
eliminating,  and  the  conservative  or  selecting  agency  of  the 
animate  and  inanimate  environment,  were  the  principal  factors 
in  evolution.  Given  a  sufficient  crop  of  indefinite  variations, 
— unanalysed  or  unanalysable  as  to  their  origin, — the  struggle 
for  existence  separated  the  minority  of  wheat  ears  from  the 
majority  of  tares,  and  secured  a  finer  and  finer  harvest. 

So  much  had  Darwin  in  his  magistral  labours  to  do  with 
making  the  general  conception  of  evolution  current  coin,  that 
we  can  readily  understand  how  not  only  the  educated  laity, 
but  the  majority  of  professed  naturalists,  identified  their 
adherence  to  the  general  doctrine  with  a  subscription  to  the 
specific  principle  of  natural  selection,  and  in  becoming  evolu- 
tionists became  at  the  same  time  Darwinians,  that  is  to  say, 
natural  selectionists.  Of  late  years,  however,  as  conflict  has 
passed  from  the  outworks  to  the  very  citadel  of  evolution, — has 
come,  that  is  to  say,  to  centre  round  the  problem  of  the  origin 
of  variations, — history  has  repeated  itself.  Naturalists  such  as 
Nageli,  Mivart,  and  Eimer  have  championed  the  cause  of 
internal  organismal  variations,  of  evolution  in  terms  of  the  con- 
stitution of  the  organism,  of  progress  according  to  the  definite 


304  THE    EVOLUTION    OF    SEX. 

laws  of  organic  growth.  An  active  school  of  neo-Lamarckians, 
such  as  Cope  and  Packard,  has  arisen  in  America ;  while 
Spencer  has  re-emphasised  the  importance  both  of  function  and 
of  environment  as  factors  in  organic  evolution,  supported  more- 
ever  in  this  position  by  the  experimental  work  of  Semper  and 
others.  The  last  published  essays  of  Spencer  may  be  referred 
to  in  illustration  of  the  unended  state  of  the  controversy,  but 
at  the  same  time  of  the  growing  tendency  to  limit  the  importance 
of  natural  selection,  and  as  a  good  instance  of  successful 
endeavour  to  recognise  the  measure  of  truth  in  the  different 
theories.  Wallace  remains  staunchest  among  the  upholders  of 
the  theory  of  natural  selection,  for  his  share  in  which  he  seems 
ever  to  refuse  to  take  to  himself  sufficient  credit ;  but  it  is 
interesting  to  notice,  that  in  his  recent  valuable  work,  in  re-in- 
forcing  his  old  objections  against  the  importance  which  Darwin 
attached  to  sexual  selection,  he  has  made  admissions  welcome 
to  those  of  us  who  believe  that  the  shoulders  of  natural  selection 
have  also  been  overburdened.  As  we  have  already  noticed, 
the  phenomena  of  male  ornament  are  discussed  and  summed 
up  as  being  "  due  to  the  general  laws  of  growth  and  develop- 
ment," and  as  such  that  it  is  "  unnecessary  to  call  to  our  aid  so 
hypothetical  a  cause  as  the  cumulative  action  of  female  pre- 
ference." Again  "  if  ornament  is  the  natural  product  and  direct 
outcome  of  superabundant  health  and  vigour," — a  view  to  which 
the  reader  of  the  preceding  pages  can  be  no  stranger, — "  then 
no  other  mode  of  selection  is  needed  to  account  for  the  presence 
of  such  ornament."  Granted,  but  does  not  the  author  see,  that 
if  the  origin  of  characters  so  important  as  those  often  possessed 
by  males  is  to  be  ascribed  to  internal  constitution  rather  than 
to  external  selection,  the  origin  of  this,  that,  and  the  other  set 
of  characters  will  next  be  explained  in  the  same  way,  as  the 
heretics  are  in  fact  now  doing.  In  pulling  down  the  theory  of 
sexual  selection  in  favour  of  that  of  natural  selection,  Mr 
Wallace  has  really  handed  over  Mr  Darwin's  elaborate  outwork 
to  the  enemy,  who  will  not  fail  to  see  its  value  for  a  new  assault. 
Before  we  conclude  this  necessary  historical  sketch,  we 
must  however  refer  to  the  subject  of  debate  recently  re-opened 
by  Weismann,  to  whom,  as  one  of  the  foremost  of  European 
naturalists,  the  reader's  attention  has  already  been  so  frequently 
directed.  To  a  very  large  extent  at  least,  we  and  our  fathers 
have  believed  that  characters  acquired  by  the  individual 
organism  from  functional  or  environmental  conditions  might  be 


THE.  REPRODUCTIVE    FACTOR    IN    EVOLUTION. 


305 


transmitted  as  a  legacy  to  the  offspring.  According  to  Weis- 
mann,  and  not  a  few  others  independent  of  and  dependent  on 
him,  this  has  been  a  delusion.  Not  only  is  positive  proof  of 
such  transmission  of  ^(r^2/z><f^  characters,  i.e.^  other  than  those  of 
constitutional,  congenital,  or  germinal  origin,  so  scanty  and 
unsatisfactory  that  His  has  not  hesitated  to  call  the  catalogue 
of  cases  a  mere  "  handful  of  anecdotes,"  but  the  connection 
between  the  body-cells  and  the  sex-elements  seems  to  Weis- 
mann  and  his  school  so  far  from  close  or  dependent,  that  there 
is  a  great  probability  against  any  "  somatic  "  dint  or  modification 


Two  adjacent  animal  cells,  showing  communications  through 
adjacent  intercellular  substance;  also  the  protoplasmic 
network,  and  the  nucleus. — After  Pfitzner. 

directly  affecting  the  reproductive  elements, — that  is  to  say, 
affecting  the  offspring.  If  the  reproductive  elements,  in  spite 
of  the  close  connection  between  all  parts  of  the  body,  or  even 
between  cell  and  cell  (see  above  fig.),  do  lead  such  a  charmed 
physiological  life  within  the  organism  that  they  are  unaffected 
directly  by  changes  in  the  other  parts  of  the  body,  then  an 
optimism  of  heredity  is  demonstrable.  How  far  we  believe 
it  from  being  so  cannot  be  here  discussed,  but  the  conse- 
quences of  Weismann's  conclusion  to  the  general  theory  of 
evolution  must  be  re-emphasised.  If  individually  acquired 
characters  are  of  importance  only  to  the  individual  body,  they 

u 


3o6  THE    EVOLUTION    OF    SEX. 

are  obviously  of  no  account  in  the  evolution  of  the  species, — 
above  the  level  of  the  Protozoa  at  least ;  and,  as  Weismann 
himself  says,  the  ground  is  thus  taken  from  under  the  feet  of 
Buffonians,  Lamarckians,  neo-Lamarckians,  &c.  The  ground 
is  left  clear  for  natural  selectionists,  and  the  struggle  for 
existence  acting  on  variations  thus  becomes  the  exclusive 
factor  in  the  mechanism  of  evolution.  But  what  then  starts 
these  variations  which  natural  selection  eliminates  or  fosters,  as 
the  case  may  be  ?  Weismann's  answer  is  clear  and  definite,  the 
intermingling  of  the  sexual  elements  in  fertilisation  is  the  sole 
fountain  of  variation ;  a  view  which  certainly  accents  the 
"  Reproductive  Factor  in  Evolution,"  though  it  seems  to  us 
hardly  to  conform  with  the  author's  previously  expounded 
opinion,  that  the  action  of  the  sperm  upon  the  ovum  is  quanti- 
tative rather  than  qualitative.  But,  even  if  none  but  constitu- 
tional or  germinal  variations  are  transmissible,  we  are  not  shut 
up  to  the  exclusive  adoption  of  the  natural  selectionist  position. 
It  is  still  open  to  the  naturalist  to  demonstrate,  that  many 
adaptations  at  least  are  not  explicable  as  the  result  of  a  long 
process  of  fostering  and  eliminating  selection  among  a  host  of 
sporadic  results  of  sexual  interminglings,  but  are  rather  the 
direct  and  necessary  results  of  "  laws  of  growth,"  of  "  constitu- 
tional tendencies,"  or  of  the  precise  chemical  nature  of  the 
protoplasmic  metabolism  in  the  organisms  in  question.  If 
constitutional  variations  occur  along  a  few  definite  lines,  as 
Eimer,  Geddes,  and  others  have  shown  to  be  true  in  certain 
cases,  then  we  can  understand  the  origin,  though  not  perhaps 
the  distribution,  of  species  apart  from  any  long  process  of  selec- 
tion, for  which  indeed,  if  variations  be  stictly  definite,  the 
material  must  be  vastly  reduced.  In  other  words,  we  can 
think  of  the  organism  not  merely  under  the  moulding  influence 
of  its  functions,  nor  solely  as  the  product  of  environmental 
hammering,  least  of  all  as  the  survivor  from  a  crowd  of  unsuc- 
cessful competitors,  but  as  the  expression  of  an  internal  fate, 
no  longer  mystical,  but  expressible  in  terms  of  the  dominant 
chemical  constitution. 

§  2.  The  Reproductive  Factor. — Without  further  discussion 
of  the  still  open  controversy  as  to  the  various  factors  of  evolu- 
tion, which  would  not  be  relevant  to  such  a  work  as  this,  we 
must  summarily  collate  the  more  prominent  opinions  as  to  the 
share  reproduction  has  in  the  process.  To  most  of  these  we 
have  already  alluded  in  the  body  of  the  book. 


THE  REPRODUCTIVE  FACTOR  IN  EVOLUTION.      307 

(a.)  First  of  all,  as  to  the  origin  of  variations,  we  find  that 
what  Treviranus  recognised  in  the  first  years  of  this  century, — 
viz.,  the  influence  of  fertilisation  in  evoking  change, — has 
been  emphasised  by  several,  such  as  Brooks  and  Galton,  and 
has  been  especially  elaborated  by  Weismann.  As  we  have 
just  seen,  Weismann  finds  in  the  intermingling  of  two  "germ- 
plasmas,"  which  is  the  essence  of  fertilisation,  the  sole  origin  of 
variations  of  any  account  in  the  evolution  of  the  species. 
Whether  this  be  consistent  with  Weismann's  theory  of  fertilisa- 
tion or  not  is  matter  for  debate,  but  there  is  no  doubt  that  his 
emphasis  on  the  evolutionary  value  of  sexual  reproduction  is  a 
most  important  contribution  to  the  general  theory.  In  some- 
what marked  contrast  is  the  view  recently  advocated  by  Hat- 
schek,  who  sees  in  the  intermingling  essential  to  fertilisation  a 
counteractive  of  idiosyncracies,  a  means  of  controlling  and 
checking  disadvantageous  individual  peculiarities.  The  two 
positions  are  not  antagonistic,  but  rather  complementary  to  one 
another. 

(/k)  No  impartial  student  of  Darwinism  can  fail  to  admit, 
that  in  the  "  struggle  for  existence "  stress  is  laid  upon  the 
nutritive  and  self-maintaining  functions  and  strivings,  while  the 
reproductive  and  species-maintaining  activities  are  regarded  as 
of  secondary  importance.  One  cannot  forget,  indeed,  how 
much  Darwin  insisted  upon  the  ro/e  of  "  sexual  selection ; "  yet 
it  has  been  already  shown  that  this  recognition  of  the  repro- 
ductive factor  was,  after  all,  very  external ;  that  sexual  selection 
is  only  a  special  case  of  natural  selection ;  that  it  seeks  to 
explain  the  elaboration,  not  the  origin  of  sexual  peculiarities ; 
and  lastly,  that  Darwin's  arguments  in  favour  of  the  mecha- 
nism which  he  emphasised,  have  been  seriously  impugned  by 
Wallace  in  an  attack  which  reacts  strongly  upon  the  critic's 
own  position. 

(c.)  Romanes  has  recently  elaborated,  what  others  seem  also 
to  have  suggested,  the  importance  of  mutual  sterility  in  splitting 
up  one  species  into  several.  "  Whenever  any  variation  in  the 
highly  variable  reproductive  system  occurs,  tending  to  sterility 
with  the  parent  form  without  impairing  fertility  with  the  varietal 
form,  a  physiological  barrier  must  interpose,  dividing  the 
species  into  two  parts,  free  to  develop  distinct  histories,  with- 
out mutual  intercrossing,  or  by  independent  variation."  The 
reproductive  system  is  very  apt  to  vary, — why,  he  does  not 
say ;    the    consequence    might    readily    be,  that   among    the 


3o8  THE    EVOLUTION    OF    SEX. 

progeny  of  a  parent  stock  some  were  fertile  inter  se,  but  infer- 
tile with  the  consistent  members  of  the  parent  stock  ;  these 
will  be  isolated  by  a  physiological  barrier,  just  as  they  might  be 
insulated  by  a  geographical  one,  and  left  free  to  develop  along 
divergent  paths  of  their  own.  Here  again  there  is  recognition 
of  the  reproductive  factor  in  evolution;  but  how  far,  and  in 
what  cases  species  have  so  originated,  is  obviously  a  ques- 
tion which  would  involve  discussion  of  each  individual 
instance. 

[d.)  Worthy  of  reiteration  is  the  suggestion  of  Robert 
Chambers,  crudely  illustrated  as  it  may  have  been,  that 
environmental  influences  acted  with  special  power  upon  the 
generative  system,  and  that  the  prolongation  of  gestation  was  a 
maternal  sacrifice  which  brought  its  own  reward  in  the  higher 
evolution  of  the  offspring.  Miss  Buckley,  along  a  similar  line 
of  thought,  has  well  pointed  out  how  the  increase  of  parental 
care  was  a  factor  in,  as  well  as  a  result  of  the  general  ascent ; 
how  the  success  of  birds  and  mammals  especially  must  in  part 
be  interpreted  in  reference  to  the  noteworthy  deepening  of 
parental  affection,  and  strengthening  of  the  organic  and  emo- 
tional links  l^etween  mother  and  offspring.  In  emphasising  the 
progressive  value  of  prolonged  infancy,  especially  in  the  evolu- 
tion of  the  emotions,  Fiske  has  also  recognised  the  importance 
of  the  reproductive  factor. 

§  3.  Further  Construction. — I'he  general  tendency  of  all 
theories  of  evolution  has  been  to  start  with  the  individual 
organism  as  the  unit,  and  to  consider  the  self-maintaining  and 
nutritive  activities  as  primary,  the  reproductive  and  species-re- 
garding as  only  secondary.  Butalong  many  lines  of  research,  such 
as  those  indicated  in  the  preceding  jjaragraph,  the  importance 
of  the  reproductive  factor  has  been  recognised,  and  the  centre 
of  gravity  of  the  evolutionary  inquiry  has  already  been  to  some 
degree  shifted.  Recent  investigations  on  heredity,  for  instance, 
forbid  that  attention  should  any  longer  be  concentrated  on  the 
individual  type,  or  reproduction  regarded  as  a  mere  re})ctition 
process  ;  the  living  continuity  of  the  species  is  seen  to  be  of 
more  importance  than  the  individualities  of  the  separate  links. 
Physiologists  and  evolutionists  are  coming  to  see  the  most 
complex  individual  lives,  in  Foster's  phrase,  as  "  but  the  bye- 
play  of  ovum-bearing  organisms."  The  species  is  a  continuous 
undying  chain  of  unicellular  reproductive  units,  which  indeed 
build   out    of   and   around   themselves   transient   multicellular 


THE  REPRODUCIIVE  FACTOR  IN  EVOLUTION.      309 

bodies,  but  the  processes  of  nutritive  differentiation,  nnd  other 
individual  developments,  are  secondary,  not  primary. 

Thus  it  is  the  central  generalisation  of  botany  that,  despite 
the  individual  differentiation  of  fern,  selaginella,  cycad,  conifer, 
and  flower,  these  turn  out,  on  deepest  analysis,  to  be  but  the 
surviving  phases  of  a  continuous  and  definite  increase  in  the 
subordination  of  the  sexual  parents  to  their  asexual  offspring 
(see  pp.  201,  211). 

Or  if  we  take  in  particular  the  origin  of  the  flower,  which 
all  botanists  agree  in  regarding  as  a  shortened  branch,  the 
natural  selectionist  explanation  (did  the  theory  trouble  itself 
with  such  questions)  would  seem  to  be,  that  the  flower  had 
arisen  by  selection  from  the  two  other  alternatives  of  lengthened 
and  unshortened  axes.  But  this  is  at  once  excluded  by  the 
physiological  explanation  that  shortening  of  the  axis  was  inevit- 
able^ since  the  expense  of  the  reproductive  functions  necessarily 
checks  the  vegetative  ones,  for  it  is  evident  that  we  cannot 
speak  of  selection  where  the  imaginable  alternatives  are  physi- 
cally impossible.  So  too  the  shortening  of  the  inflorescence 
from  raceme  to  spike  or  flowerhead,  or  still  further  into  the 
hollowed  form  of  a  fig,  with  the  corresponding  reduction  in  the 
size  of  the  flowers,  is  again  the  result  of  the  check  imposed  by 
reproduction  on  the  growth  of  axis  and  appendages. 

The  same  simple  conception  of  a  continuous  checking  of 
vegetation  by  reproduction,  unlocks  innumerable  problems  of 
floral  structure,  large  and  small  alike,  from  the  inevitable 
development  of  gymnosperm  intoangiosperm  by  the  continuous 
subordination  of  the  reproductive  carpellary  leaf,  to  the  varia- 
tions of  cabbages  as  seen  in  the  transitions  between  leafy  kale 
and  cauliflower.  Or  again,  the  origin  of  floral  colour,  as 
primarily  an  inevitable  consequence  of  the  same  principle  of 
vegetative  subordination  through  reproductive  sacrifice,  was 
long  ago  pointed  out  by  Spencer,  and  admits  of  detailed  elabora- 
tion without  attaching  more  than  secondary  importance  to 
selection  by  insects. 

In  another  way,  the  antithesis  between  reproduction  and 
nutrition  may  be  illustrated  among  the  existing  orders  and 
species  of  flowering  plants.  Just  as  the  lilies,  for  instance, 
range  on  the  one  side  towards  the  characteristically  vegetative 
grass,  or  on  the  other  towards  the  reproductive  orchid,  so  it  is 
with  the  main  variations  of  every  natural  alliance.  Thus,  the 
Ranunculacese  have  their  grassy  and  their  orchid-like  types  in 


3IO 


THE    EVOLUTION    OF    SEX. 


meadow-rue  and  larkspur  respectively,  while  the  species  of  these 
very  genera  show,  within  narrower  limits,  similar  swings  of 
variation.  AVhat  we  call  higher  or  lower  species  are  thus  the 
leaders  or  the  laggards  along  one  or  other  of  these  two  lines  of 
variation. 

Among  animals,  the  importance  of  the  reproductive  factor 
may  be  illustrated  in  the  most  diverse  series.  Thus  the  greatest 
step  in  organic  nature,  that  between  the  single-celled  and 
many-celled  animals,  bridged  as  it  is  by  loose  colonies  some 
of  which  are  at  a  very  low  morphological  level,  is  not  due  to 


Formation  of  the  Gastnila. — From  Ha;ckel. 


the  selection  of  the  more  individuated  and  highly  adapted 
forms,  but  to  the  union  of  relatively  unindividuated  cells  into 
an  aggregate,  in  which  each  becomes  diminishingly  competitive 
and  increasingly  subordinated  to  the  social  whole.  The 
colonial  or  multicellular  forms,  originating  pathologically  in  all 
probability,  may  of  course  have  rapidly  justified  their  existence 
in  the  struggle  for  existence,  just  as  unions  of  many  kinds  do 
in  human  society,  but  the  Protozoa  cannot  be  accused  of  any 


THE    REPRODUCTIVE    FACTOR    IN    EVOLUTION.'  3rT 

prevision  of  future  advantage  in  remaining  clubbed  together  in 
co-operation,  nor  indeed  credited  with  much  primitive  altruism 
in  so  doing.  None  the  less  is  it  clear,  that  this  greatest  of 
morphological  steps  was  directly  due,  not  to  any  struggle, 
but  rather  to  an  organic  sociality,  or  at  any  rate  to  a  process 
which  is  not  interpretable  in  terms  of  individual  advantage. 

No  structure  is  more  emphatically  nutritive  in  its  adult 
result  than  the  gut-cavity  of  the  embryonic  gastrula.  It  is 
worth  inquiring  whether  this  important  step  in  differentiation 
was  attained  in  history  in  response  to  nutritive  needs.  The 
usual  supposition  is  certainly  that  the  gastrula  cavity,  by  what- 
ever peculiarities  of  growth  it  may  have  arisen,  justified  itself 
from  the  first  in  an  additional  nutritive  advantage.  But 
Salensky,  in  his  studies  on  the  primitive  form  of  the  Metazoa, 
has  given  strong  arguments  in  favour  of  the  theory  that  the 
primitive  cavity,  arising  in  a  volvox-like  form,  was  originally  a 
brood-cavity  or  "genitoccel,"  and  that  it  only  secondarilyacquired 
nutritive  significance.  It  would  be  indeed  striking  if  this 
important  morphological  step  in  the  establishment  of  the 
nutritive  system  was  reached  along  the  road  of  reproductive 
modification ;  for  if  this  most  fundamental  of  nutritive  and 
self-maintaining  advantages,  the  belly  itself,  be  but  a  secondary 
resultant  of  an  originally  reproductive  and  species-regarding 
progress,  that  lower  Utilitarianism,  which  has  so  long  been 
arguing  from  economics  to  biology  and  back  again,  is  evidently 
a  step  nearer  exposure. 

Or  again,  that  increase  of  reproductive  sacrifice,  which  at 
once  makes  the  mammal  and  marks  its  essential  stages  of 
further  progress  through  oviparous  monotreme,  prematurely- 
bearing  marsupial,  and  various  grades  of  placental ;  that 
increase  of  parental  care ;  that  frequent  appearance  of  sociality 
or  co-operation,  which  even  in  its  rudest  forms  so  surely 
secures  the  success  of  the  species  attaining  it,  be  it  mammal  or 
bird,  insect  or  even  worm, — all  these  phenomena  of  survival  of 
the  truly  fittest,  through  love,  sacrifice,  and  co-operation, 
need  far  other  prominence  than  they  could  possibly  receive  on 
the  hypothesis  of  the  essential  progress  of  the  species  through 
internecine  struggle  of  its  individuals  at  the  margin  of  subsist- 
ence. Each  of  the  greater  steps  of  progress  is  in  fact  associated 
with  an  increased  measure  of  subordination  of  individual  com- 
petition to  reproductive  or  social  ends,  and  of  interspecific 
competition  to  co-operative  association. 


312 


THE    EVOLUTION    OF    SEX. 


The  corresponding  progress  in  the  historic  and  individual 
world,  from  sex  and  family  up  to  tribe  or  city,  nation  and  race, 
and  ultimately  to  the  conception  of  humanity  itself,  also 
becomes  increasingly  apparent.  Competition  and  survival  of 
the  fittest  are  never  wholly  eliminated,  but  reappear  on  each 
new  plane  to  work  out  the  predominance  of  the  higher,  i.e.^ 
more  integrated  and  associated  type,  the  phalanx  being 
victorious  till  in  turn  it  meets  the  legion.  But  this  service  no 
longer  compels  us  to  regard  these  agencies  as  the  essential 
mechanism  of  progress,  to  the  practical  exclusion  of  the 
associative  factor  upon  w^hich  the  victory  depends,  as  economist 
and  biologist  have  too  long  misled  each  other  into  doing.     For 


An  Opossum  {Dldelphys  dorsigera   carrying  its  young  on  its  back. — 
From  Carus  Sterne 

we  see  that  it  is  possible  to  interpret  the  ideals  of  ethical  pro- 
gress, through  love  and  sociality,  co-operation  and  sacrifice, 
not  as  mere  Utopias  contradicted  by  experience,  but  as  the 
highest  expressions  of  the  central  evolutionary  process  of  the 
natural  world.  The  ideal  of  evolution  is  indeed  an  Eden  ; 
and  although  competition  can  never  be  wholly  eliminated, 
and  progress  must  thus  approach  without  ever  completely 
reaching  its  ideal,  it  is  much  for  our  pure  natural  history  to 
recognise  that  "  creation's  final  law "  is  not  struggle  but  love. 
The  fuller  working  out  of  this  thesis,  however,  would  lead  us 
far  beyond  our  present   limits,  towards  a  restatement  of  the 


THE  REPRODUCTIVE  FACTOR  IN  EVOLUTION.      313 

entire    theory  of  organic   evolution.      Leaving   this    to   other 
papers,  but  specially  to  a  future  work,  suffice  it  here,  in  con- 
clusion, to  indicate  an  important  change  in  the  general  point  of 
view.      The  older  biologists    have   been  primarily  anatomists, 
analysing  and  comparing  the  form  of  the  organism,   separate 
and  dead ;  however  incompletely,  we  have  sought  rather  to  be 
physiologists,  studying  and  interpreting  the  highest  and  intensest 
activity  of  things  living.     From  the  study  of  individual  structure 
they    were    wont    to    pass,    indeed,    to    that   of    reproductive 
structures,  and   thence   even  functions ;   hence,  too,  the  pair 
and  the  totality  of  the  species  did  at  length  come  successively 
into  view ;    but  this  with  the  individualistic  theory  of  natural 
selection  bulking  as  practically  all-important  in  the  foreground, 
to  which  even  sexual  selection  was  a  mere  harmonious  corollary. 
For  us,  however,  this  perspective  has  become  entirely  reversed. 
The  individual  is  a  mere  link  in  the  species,  and  its  repro- 
ductive processes  are  thus  of  fundamental  importance  to  the 
interpretation  even  of  its  self-maintaining  ones.     Hence  we  no 
longer  regard,  with   Darwin  and  the  majority  of  our  brother 
naturalists,  the  operation  of  natural  selection  upon  individual 
characters  as  the  simplest  of  problems,  looking   for  residual 
explanation  to  sexual  selection,  and  only  in  extreme  difficulty 
invoking    the   aid    of    "  principles    of  correlation,"    "  laws    of 
growth,"  and  the  like,  viewed  as  almost  inscrutably  mysterious. 
On  the  contrary,  it  is  the  continual  correlation  yet  antithesis — 
the  action  and  reaction — of  vegetative  and  reproductive  pro- 
cesses   in    alternate    preponderance,    which    seems    to    us    of 
fundamental  importance,  since  to  this  the  general  rhythm  of 
individual  and  racial  life  runs  fully  parallel.     Hence  it  is  that 
we  have  the  primeval  lily   developing  on  the  one  hand   the 
ideally  vegetative  grass,  yet  also  the  supremely  specialised  re- 
productive orchid  ;  and  that  we  can  trace  (as  we  hold)  the  same 
swing  of  divergent   evolution,  of  definite  variation,  in  every 
natural   order,  nay,  in    every  genus,  often    even    in  the  very 
varieties  of  a  species.    Hence,  too,  it  is  that  the  rhythm  of  hydroid 
and  medusoid  in  the  individual  life  of  the  typical  forms  becomes 
fixed  in  coral  or  ctenophore  as  a  racial  temperament.     This 
preponderance    of  passivity   or   activity   (which  we    can    read 
throughout,  in  barnacle  and  insect,  as  well  as  in  tortoise  and 
swallow)  once  set  up,  goes  on  accumulating  till  it  meets  rever- 
sal through   environment  or  other   causes,  and  limitation  or 
extinction    through    the   agency   of    natural    selection,   which, 


314  THE    EVOLUTION    OF    SEX. 

however,  is  more  frequently  a  retarding  force  than  an  accel- 
erant of  evolution.  The  problem  of  organic  progress  is  thus 
to  be  interpreted  not  merely  as  on  conventional  lines,  by  help  of 
an  analogy  derived  from  an  age  of  mechanical  progress  which 
gives  us  the  watch,  or  sewing-machine,  or  tricycle, — by  the 
cumulative  patenting,  as  it  were,  of  useful  improvements  in 
detail.  The  essential  problem  is  not  one  of  mechanism  but 
of  character,  to  which  incident  is  accessory  but  not  fundamen- 
tal,— not  of  details  put  together,  but  of  aggregate  organic  life  or 
temperament.  The  life  of  the  individual  or  the  species  is 
essentially  a  unity,  of  which  the  specific  characters  are  but 
the  symptoms,  be  their  subsequent  measure  of  importance 
and  utility  in  adaptation,  their  modification  by  environment, 
their  enhancement  or  diminution  by  natural  selection,  what  it 
may.  Our  special  study  of  the  reproductive  process  has  thus 
fairly  brought  us  to  the  threshold  of  a  larger  inquiry,  the 
primary  one  of  the  organic  sciences,  that  of  the  factors  of 
organic  evolution.  For  it  is  in  nature,  as  Schiller  saw  long  ago 
in  the  human  life,  which  this  foreshadows:  "While  philosophers 
are  disputing  about  the  government  of  the  world.  Hunger  and 
Love  are  performing  the  task." 


THE  REPRODUCTIVE  FACTOR  IN  EVOLUTION.      315 


SUMMARY. 

1.  A  brief  review  of  the  history  of  evolution  theories,  and  of  the 
present  state  of  the  question. 

2.  A  reproductive  factor  in  evolution  has  been  hinted  at  by  a  few 
naturalists. 

3.  Further  indications  of  the  importance  in  evolution  of  reproductive 
and  species-regarding,  as  opposed  to  the  nutritive  and  self-maintaining 
activities. 


LITERATURE. 

See  articles  by  the  writers  in  *'  Chambers's  Encyclopaedia,"  especially 
Biology,  Botany,  Environment,  Evolution,  and  minor  articles, 
such  as  CcELENTERATES,  Flower,  Fruit,  &c.  ;  also  "  Encyclopcedia 
Britannica,"  Sex,  and  Variation  and  Selection  ;  also  Geddes,  A 
Restatement  of  the  Theory  of  Organic  Evolution  (Summary  in  Proc. 
Roy.  Soc,  Edin.,  1888-S9,  still  unpublished). 

Spencer,  Mivart,  Eimer,  Wallace,  Weismann,  &c.— 0/.  cit. 


INDEX. 


Age  of  parents  influencing  sex,  34, 

35 
Algae,  reproduction  in,  126,  127 
Allanlois,  249 
Aloe,  243 
Alternation  of  generations,  200-215, 

229,  230 
Alternation  of  generations  in  plants, 

201,  210,  211 

Altruism,  development  of,  279-2S1 

Amnion,  249 

Amphibians,  parental  care  of,  253, 

254  ;  amatory  emotions  of,  265 
Anabolism,  see  Protoplasm 
Angiostomum,  71,  208 
Animalculists,  83,  84,  157,  158 
Anther  cells,  crystals  in,  131 
Aphides,  45,  46,  169,  172,  173,  175, 

225,  283 
Apospory,  206 
Archoplasm,  98 
Argonauta,  246,  247 
Arthropods,       hermaphroditism 

among,  72 
Ascaris,  ovum  of,  145,  146 
Asexual  reproduction,  188-199 
Aura  seminalis,  158,  159 
Aurelia,  alternation  of  generations, 

202,  203 

Baer,  Von,  discovery  of  mammalian 

ovum,  86 
Balbiani    on  isolation   of  sex-cells, 

92  ;  conjugation  of  Protozoa,  149 
Balfour  on  polar   bodies,    106 ;    on 

parthenogenesis,  180 
Barry,  Martin,  142 
Bary,  De,  on  fertilisation,  160 


Bees,  sex  characters,  42 ;  partial 
parthenogenesis,  172;  reproduc- 
tion in,  185,  186 

Beneden,  Van,  on  origin  of  sex- 
cells,  91  ;  on  polar  bodies,  106  ; 
on  fertilisation,  145 

l^ichat's  analysis,  86 

Bilharzia,  71,  265 

Bird  of  Paradise,  4 

Birds,  sexual  characters,  6,  7 ;  sexual 
selection  in,  ii  ;  eggs,  103  ;  love 
among,  266,  267 

Blackcock,  7 

Blochmann  on  polar  bodies,  105, 
180 

Body  versus  reproductive  cells,  93, 
260-262 

lionellia,  20 

Boveri  on  archoplasm,  98  ;  on  fer- 
tilisation, 145-148,  160 

Brood-chambers.  255 

Brooding,  63 

Brooks  on  sexual  selection,  10-12; 
theory  of  sex,  118,  132;  on  fer- 
tilisation, 166 

Biichner  on  love  among  animals, 
266 

Biitschli  on  extrusion  of  polar  glo- 
bules, 106 

Buffon,  300 

Butterflies,  28,  46 

Canestrini,  theory  of  sex,  34 

Carnoy  on  fertilisation,  145,  146 

Castration,  effects  of,  23 

Cecidomyia,  243 

Cell-cycle,  118 

Cell-division,  221-223,  233,  234,  256 


3i8 


INDEX. 


Cell  theory,  86 

Chara,  reproductive  organs  of,  131 

Chironomus,  separation  of  sex-cells 
in,  92,  173 

Chambers,  Robert,  on  prolonged 
gestation,  302,  308 

Claspers,  62,  247 

Coccus  insects,  17,  20 

Coelebogyne,  175 

Coelenterates,  reproductive  organs, 
59,  60 ;  hermaphroditism,  70  ; 
origin  of  sex-cells,  91  ;  asexual 
reproduction,  193-195  ;  alterna- 
tion of  generations,  209 

Colour,  animal,  23,  24 

Colour,  floral,  309 

Comparative  vigour,  theory  of,  36 

Conjugation,  151,  152,  161 

Conjugation,  multiple,  151 

Continuity  of  cells,  305 

Continuity  of  germ-plasma,  see 
Weismann 

Copulation,  245-247 

Co-operation,  311 

Crime  among  animals,  275.  See 
Cuckoo 

Crustaceans,  sex  in,  46  ;  partheno- 
genesis in,  174 

Cuckoo,  habits  of,  274-279 

Cupid's  dart,  247 

Cutleria,  reproduction  in,  128 

Cuttlefishes,  organs  of,  62  ;  fertilis- 
ation in,  62  ;  egg-clusters  of,  274 


Darwin  on  sexual  selection,  8-14, 
25,  27,  29  ;  on  determination  of 
sex,  38  ;  on  cross  and  self  ferti- 
lisation, 138;  on  population  ques- 
tion, 290,  291  ;  natural  selec- 
tion, 303,  304 

Darwin,  Erasmus,  300,  301 

Darwinism,  312-314 

Death,  origin  of,  255,  260,  261 

Dichogamy,  73 

Diplozoon,  246,  247 

Division  of  labour,  193 

Drones,  nature  of,  19 

Ducts  of  reproductive  organs,  60 

Diising  on  the  proportions  of  the 
sexes,  38  ;  on  sex-determination, 
47 


Echinodermata,  hermaphroditism 
among,  72 

Ectocarpus,  reproduction  in,  128 

Edible  bird's  nest,  250 

Egg-cell,  see  Ovum 

Egg,  chemistry  of  the,  104 

Egg-envelopes,  102 

Egg-laying  organs,  62 

Egg-shell,  103,   104 

Egoism,  279-281 

Eimer  on  humble  bees,  44  ;  on 
cuckoo,  279  ;  on  evolution,  306 

Embryology,  82-86,  90-92,  97-105, 
II2-114 

Encystation,  193,  259 

Engelmann  on  dimorphism  in  Pro- 
tozoa, 129 

Environment,  influence  of,  235,  236; 
transmission  of  acquired  char- 
acters, 304,  306 

Epigenesis,  82-86 

Ethical  aspects  of  nutrition  and  re- 
production, 279-281 

"  Evolution"  in  old  sense,  82-84 

Evolution,  theories  of,  300-305  ;  re- 
productive factor  in,  300-315 ; 
essential  problems  of,  313,  314 

Fallopian  tul)e,  242 

Females,  1-50  passim;  passivity, 
17  ;  size,  21  ;  preponderant  ana- 
bolism,  26  ;  psychical  characters 
of,  267-271 

Fern,  alternation  of  generations, 
205 

Fertilisation,  time  of,  34  ;  in  plants, 
138;  details  of,  144-148  ;  nuclear 
union  in,  145  ;  duality  of,  148  ; 
in  Protozoa,  149  ;  origin  of,  150- 
153  ;  theory  of,  157-168  ;  uses  of, 
163-167;  a  source  of  variation, 
301 

Fish,  sex  differences  in,  6,  30  ; 
parental  care  of,  254,  255  ;  ama- 
tory emotions  of,  265 

Flower,  position  of,  226  ;  origin  of, 

309- 
Fluke,    alternation    of  generations, 

204,  207 
Follicular  cells,  103 
Free-martin,  37 
Fungi,  parthenogenesis  in,  176 


INDEX. 


319 


Gall-wasps,  parthenogenesis  in,  175 
Gallon,  contrast  between  body  and 

sex  cells,  93  ;  on  fertilisation,  163 
Gastrula,  85,  310,  31 1 
Gegenbaur  on  origin  of  yolk  gland, 

61  ;  on  hermaphroditism,  78 
Gemmules   of    fresh-water  sponge, 

209 
Genesis,  285 

Gentry  on  sex  in  moths,  46 
Germiparity,  33,  66 
Germ-plasma,  see  Weismann 
Gestation,  length  of,  247;  influence 

of  prolonged,  302 
Girou  on  sex,  34,  47 
Goette  on  origin   of  death,  234  ;  on 

nemesis  of  reproduction,  255 
Gonads,    or    essential    reproductive 

organs,  57-64 
Graaf,  83 

Graafian  follicle,  242 
Growth,  219-230 
Gruber  on  reproduction  of  Protozoa, 

228 
Gyrodactylus,  207,  243 

Haeckel  on  continuity  of  repro- 
duction, 93  ;  on  Protomyxa,  120  ; 
on  alternation  of  generations, 
214;  on  Magosphaera,  255 

Haller,  83,  84. 

Hamm,  discovery  of  spermatozoon, 
109 

Harvey,  82 

Hatschek  on  fertilisation,  167  ;  on 
nutrition  and  reproduction,  227 

Hectocotylus,  62,  246 

Hensen,  theory  of  sex,  34  ;  on  fer- 
tilisation, 163 

Heredity,  see  Weismann  ;  in  alter- 
nating generations,  210,  211  ; 
acquired  characters,  304-306 

Hermaphroditism,  65-79;  embry- 
onic, 65  ;  casual,  66  ;  partial,  67; 
normal,  70  ;  degrees  of,  72  ;  con- 
ditions of,  77  ;  origin  of,  78 

Hertwig  on  hermaphroditism,  68  ; 
on  fertilisation,  159 

Heyer  on  sex  in  plants,  47. 

Hofacker  and  Sadler's  law,  34,  36 

Hofacker  on  determination  of  sex, 
34 


Hunger  and  love,  279-281 
Hybridisation,  153-155 
Hydra,  59,  189,  225 
Hydractinia,  193 

Hydroids,  alternation  of  genera- 
tions, 201,  202 

Immortality,  organic,  258-262 

Incubation,  251-255 

Individuation,  285-287,  291,  292 

Insects,  sex  characters  of,  5,  1 1, 
16-19  5  determination  of  sex  in, 
42-46 ;  hermaphroditism  of,  69- 
70;  parthenogenesis  in,  174,  175 

Isophagy,  161 

Jteger  on  continuity  of  germ-proto- 
plasm, 93 

Katabolism,  see  Protoplasm 
Knight,    experiments     on      water- 
melons, 48,  49 
Kolliker   on  origin  of  spermatozoa, 
no 

Lactation,  249,  250 
Lamarck,  301 

Laulanie  on    embryonic    hermaph- 
roditism, 33,  66 
Leeuwenhoek,  83,  no,  158 
Limit  of  growth,  220-224 
Liverwort,  227 
Love,  264-281 
Love  for  offspring,  271-274. 
Luciola,  25 

Magosphrera,  255 

Males,  1-50  passim;  variability, 
12,  13;  activity,  17;  size,  20; 
pigmy,  17,  20,  75,  77;  colour, 
23  ;  predominant  katabolism, 
26  ;  complemental,  75,  77  ;  psy- 
chical characters,  304 

Male-cell,  see  Spermatozoon 

Malpighi,  %Z 

Malthus,  284,  290,  291 

Malthusianism,  290-292 

Mammals,  love  among,  266,  267 

Mammary  glands,  249,  250 

Mantegazza  on  sexual  characters, 
10,  14 

Marsupials,  252,  253 


320 


INDEX. 


Maternal  sacrifice,  256,  257 

Maupas  on  conjugation  and  multi- 
plication of  infusorians,  149,  150, 
228,  229,  235,  236  ;  on  uses  of 
fertilisation,  163-166 ;  on  theory 
of  organic  immortality,  260 

Mayflies,  257,  258 

Meehan,  investigations  on  sex  of 
plants,  47,  49  ;  on  cross-fertilisa- 
tion, 75 

Menstruation,  244,  245 

Mesozoa,  liberation  of  germs  in,  60, 

255 
Metabolism,  see  Protoplasm 
Micropyle,  103 
Microstomum,  195,  225 
Miescher   on    the  milt  of   salmon, 

Migration  of  sex-cells,  60 
Milk,  250 
*Minot    on     polar     globules,     106  ; 

theory  of  genoblasts,  118,  132;  on 

parthenogenesis,  180,  181 
Mivart  on  sexual  selection,  13,  14 
Molluscs,  hermaphroditism  among, 

72 
Molothrus,  278 
Moonwort,  226,  227 
Moss,    alternation    of    generations, 

201,  205,  206 
Myzostomata,  hermaphroditism    of. 

Natural  selection,  a  check  on  diver- 
gence of  sexes,  30  ;  scope  of,  303, 

307,  311,  313.  314 
Nematodes,     alternation    of  sexual 

generations  in,  208 
Neo-Malthusianism,  292-298, 
Notodelphys,  253 
Nototrema,  253 
Nucleus,   behaviour  in  fertilisation, 

145  ;  essential  elements  of,  240 
Nussl)aum,  94,  160 
Nutrition,  influence  on  sex,  41 
Nutrition      versus       reproduction, 

223-225 
Nutrition  of  young,  248,  249 

Obstetric  frog,  253,  254 
Organs  of  reproduction,  57-64 
Orthonectids,  bursting  of  female,  257 


Oviparous  animals,  248 

Oviparous  mammals,  248 

Ovipositors,  62 

Ovists,  83,  84,  157 

Ovo-viviparous  animals,  248 

Ovulation,  242,  243 

Ovum,  97-108;  envelopes  of,  103; 
maturation  of,  104-107;  libera- 
tion of,  242,  243 

Ovum  theory,  82 

Owen  on  somatic  and  reproductive 
cells,  93  ;  on  residual  spermatic 
force,  212 

Pa^dogenesis  or  juvenile  partheno- 
genesis, 173 

Pairing,  early  forms  of,  265 

Paper  nautilus,  brood-shell  of 
female,  254,  255 

Paramcecium,  conjugation  of,  149 

Parental  care,  270-274,  312 

Parental  sacrifice,  255,  256 

Parthenogenesis,  169-187 

Parturition,  247,  248 

Penis,  61,  247 

Pfliiger  on  polyspermy,  34  ;  on  sex 
in  tadpoles,  41 

Pigments,  expressions  of  disruptive 
processes,  24 

Pigeon's  milk,  250 

Placenta,  248 

Planarians,  multiplication  of,  229 

Plants,  determination  of  sex  in,  47, 
48  ;  origin  of  sex  among,  127  : 
conjugation  in,  143  ;  fertilisation 
in,  140,  159;  parthenogenesis  in, 
175,  176;  asexual  reproduction 
of,  191,  192 

Plasmodium,  150,  151 

Platner  on  sex-cells  in  snail,  125 

Ploss  on  embryonic  hermaphrodi- 
tism, 33,  66 

Polar  globules,  105,  106,  179-1S5 

Pollen-grain,  229,  230 

Polyspermy,  34 

Polystomum,  243 

Population  question,  289-298 

Precocious  reproduction,  207,  243, 

244 
Preformation,  theory  of,  %}i^  84 
Primary  sexual  characters,  3 
Protococcus,  127 


INDEX. 


321 


Pi'otomyxa,  150 

Protoplasm,  metabolism  of,  86,  8y, 
122,    123;    in   fertilisation,    159, 

160  ;  mechanics  of,  223  ;  in  rela- 
tion to  cell-division,  223,  224 

Protozoa  contrasted  with  Metazoa, 
57,  88,  89 ;  illustrating  cell- 
phases,  119-121;  conjugation  of, 
148- 15 1  ;  asexual  multiplication, 
192  ;  immortality  of,  90,  166, 
258,  261  ;  alternation  of  genera- 
tions in,  208 

Puberty,  241,  242 

Purkinje,  86 

Pycnogonid,  male  with  young,  273 

Rate  of  increase,  283,  284 

Rate  of  reproduction,  283-289 

Rauber  on  somatic  and  reproduc- 
tive cells,  93 

Regeneration,  189,  190 

Reproduction,  processes  of,  137- 
217;  theory  of,  221-263;  origin 
of,  232,  233  ;  in  relation  to  en- 
vironment, 234-236 ;  nemesis  of, 
234,  255 

Reproductive  cells,  81-132  passim  ; 
in  relation  to  the  "body,"  261 

Reptiles,  amatory  emotions  of, 
265,  266 

Richarz,  theory  of  sex,  37 

Rhinoderma,  254 

Rhythms  of  life,  224,  225 

Rolph  on  sex  characters,  25  ;  on  sex 
in  wasps,  44 ;  on  crustaceans,  46 ; 
theory  of  sex,  117;  on  sex-cells, 
125,    133  ;    on   fertilisation,    160, 

161  ;  on  parthenogenesis,  181 
Romanes     on    cuckoo,    275 ;      on 

physiological  selection,  307,  308 
Rotifers,    sexes   of,   20 ;    partheno- 
genesis in,  174  ;  male,  257 

Sabatier,  theory  of  polarities,  106 

Sacrifice,  311 

Sachs,  origin  of  fertilisation,   153; 

nature  of  fertilisation,  160 
Sadler  on  determination  of  sex,  34 
Salensky  on  primitive  Metazoa,  31 1 
Schizogenes,  multiplication  of,  232 
Schlechter  on  sex  in  horses,  49 
Schleiden,  86 


Schultze,  hypothesis  of  male  and 
female  ova,  34 

Schwann,  86 

Sea-anemones,  asexual  multiplica- 
tion, 188,  189 

Sea-cucumber  and  offspring,  272 

Sea-horse,  parental  care  of  male, 
,  254,255 

Secondary  sexual  characters,  3-8, 
22-24 

Self-fertilisation,  73 

Seminal  vesicles,  241 

Sex,  determination  of,  32-54 ; 
theory  of,  117-133;  origin  of, 
126 

Sexes,  characters  of,  16-25;  different 
habits,  16-19 ;  sizes  of,  19-22 ; 
intellectual  and  emotional  differ- 
ences, 267-271 

Sex  elements,  81-95  5  early  separa- 
tion of,  91  ;  compared  with  Pro- 
tozoa, 119 

Sexual  attraction,  266,  267 

Sexual  maturation,  241,  242 

Sexual  organs  and  tissues,  57-63 

Sexual  reproduction,  137-159 

Sexual  selection,  8-14,  307 

Sexual  union,  245-247 

Siebold,  Von,  experiments  on  wasps, 
44,  parthenogenesis,  170 

Silkmoth,  parthenogenesis  of,  169 

Simon  on  fertilisation,  161 

Siphonophora,  194 

Snail,  reproductive  specialisation  in 
the,  62 

vSpencer,  theory  of  growth,  220-224  ; 
laws  of  multiplication,  284-289  ; 
population  question,  288-292 ; 
factors  of  evolution,  304 

Spermatogenesis,  theory  of,   113 

Spermatozoon,  109-115;  discovery 
of,  109;  structure,  no;  forms, 
III  ;  physiology  of,  in  ;  resem- 
blance to  pollen,  114  ;  chemistry 
of,  115  ;  influence  of,  301 

Spiculum  amoris,  62 

Sponge,  reproductive  cells,  58,  59  ; 
hermaphroditism,  70  ;  asexual 
reproduction,  188,  189,  192,  193  ; 
alternation  of  generations  in  Spon- 
gilla,  208,  209 

vSpores,  205-214 


INDEX. 


Sprengel  on  fertilisation  in  flowers, 

138 
Starfish,  reproduction  of  parts,  197 
Starkweather's  law  of  sex,  36,  37 
Statistics  on  male  and  female  births, 

Steenstrup  on  alternation  of  genera- 
tions, 200 

Stickleback,  habits  of,  24 ;  secretion 
of,  250,  251 

Sterility,  298 

Strasburger  on  fertilisation,  159, 
160  ;  on  parthenogenesis,  181 

Summer  eggs,  174 

Surinam  toad,  63,  252,  253 

Sutton  on  embryonic  hermaphro- 
ditism, 33,  66 

Syllids,  asexual  multiplication,  197 

Tadpoles,  41,  248 
Tapeworm,  life-history  of,  209,  210 
Temperance,  297,  298 
Temperature,  influence   of,  on  sex, 

48,  49 
Thury,  theory  of  sex,  34 
Tiger-lily,  226 
Treviranus,    301  ;    on    influence  of 

sperm,  166,  307 
Tunicates,    asexual    multiplication, 

198  ;  alternation  of  generations, 

203 
Twins,  210  ;  sex  of,  39 

Ulothrix,  reproduction  in,  127 

Variation,  13,  166,  167,  301,  306 
Vines   on    reproduction    in    plants, 

126,  127 
Viviparous  animals,  248 


Vorticella,    reproduction    of,     129, 

149,  150 
Volvox,  58,  128-130 

Wallace,    sexual  selection,    10-14; 

natural  selection,  29,  303,  304 
Ward,  Marshall,  on  parasitic  fungi, 

229 
Water-fleas,      parthenogenesis     in, 

Weeping  willows,  188 

Weismann  on  fertilisation,  161, 
162  ;  on  polar  bodies,  105,  107  ; 
on  use  of  fertilisation,  164  ;  on 
parthenogenetic  ova,  181- 185  ; 
hydroids,  91,  210,  211 ;  alternation 
of  generations,  214;  continuity  of 
germ-plasnia,  94,  95,  239-241  ; 
on  organic  immortality,  258-261  ; 
inhericance  of  acquired  characters, 
51,  304-306  ;  variation,  306 

Whelk,  cannibalism  of  young,  249 

Winter  eggs,  174 

Wolff,  reassertion  of  epigenesis,  85 

Women,  subjection,  rights,  func- 
tions of,  267-271 

Worms,  hermaphroditism  in,  71  ; 
asexual  reproduction,  195-197  ;  al- 
ternation of  generations,  209 

Yolk  glands,  61 

Yolk-sac,  249 

Yung  on  sex  in  tadpoles,  41 

Zacharias  on  male  and  female  ele- 
ments, 114;  on  asexual  multipli- 
cation, 225 

Zona  pellucida,  103 

Zona  radiata,  103 


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before  Chaucer  to  the  present  time)  have  been  made  with  admirable  critical 
insight.  English  verse  is  rich  in  utterances  of  the  poets  about  music,  and 
merely  as  a  volume  of  poetry  about  music  this  book  makes  a  charming 
anthology.  Three  sonnets  by  Mr.  Theodore  WaUs,  on  tlie  "  Fausts  "  of 
Berlioz,  Schumann,  and  Gounod,  have  been  written  specially  for  this  volume. 
It  is  illustrated  with  designs  of  various  musical  instruments,  etc, ;  autographs 
of  Rubenstein,  Dvorak,  Greig,  Mackenzie,  Villiers  Stanford,  etc.,  etc. 

"  To  musical  amateurs  this  will  certainly  prove  the  most 
attractive  birthday  book  ever  published." — Manchester  Guardian. 

"One  of  those  happy  ideas  that  seems  to  have  been  yearning 
for  fulfilment.  .  .  .  The  book  ought  to  have  a  place  on  every 
music  stand." — Scottish  Leader. 

New  York  :  .Scribner  &  Welford. 


